Blast Cabinet Part 3 - Function

Copyright © 2005 Dave Propst. All rights reserved.

Revised 6/01/2005

Introduction

This article is the third and final article in a three-part series of articles describing the large abrasive blast cabinet seen in the picture below.

Blast Cabinet Part 1

Blast Cabinet Part 2 - Details

Blast Cabinet Part 3 - Function

The first article contains pictures and detailed descriptions of all aspects of the overall cabinet. The second article is a collection of close-up pictures of various design details of the abrasive blast cabinet. This, the third article, contains a lengthy and very highly detailed explanation of media blasting sheetmetal automotive body panels.

This article provides detailed information about media blasting of sheetmetal in general-- meaning not just the process used with this cabinet. While information about media blasting is commonplace, information about media blasting of sheetmetal is not commonplace. This article does not necessarily provide all information needed to make every decision about choosing blast media and blast process. However, this sheetmetal-specific information can be used as a 'filter' when studying more detailed but non-sheetmetal-specific data provided by abrasive media manufacturers and dealers.

Some information in this article appears in more than one section. While such repeated information at times may seem redundant, it is usually stated in a slightly different manner that is more relevant to that particular section's subject matter. This is done at times instead of directing the reader back to some previous section of the article.

In reading this or any other information about media blasting, realize that the subject of abrasive media is a complex one. How rapidly an abrasive achieves the desired result and how it affects the metal structure under whatever substance is being stripped off depends on many factors. Size, weight, hardness, and sharpness of the abrasive material, nature of the material being stripped, air pressure used, volume of air used and any number of other things come into play. Therefore it is important to not accept any generalized, sweeping statements that do not specify what substance is being removed, what it is being removed from, what abrasive is being used and what blaster settings are being used. It is mandatory to not come to conclusions about one specific blast media or blasting method and mistakenly apply those conclusions to all blast media and methods.

Uses of this Blast Cabinet

This blast cabinet is primarily used for removing rust, weld slag, and similar substances from steel automotive body panels. (The body panels being referred to in this article are those made of standard, un-plated, uncoated steel as found on most 1900 to 1970's manufactured cars and custom-built bodies of all eras.) Suspension and frame parts that have been previously stripped of heavy chassis paint and undercoating are also finish stripped of rust and cleaned in the cabinet. Newly fabricated tubular structures and other items are often cleaned of weld slag, HAZ discoloration, and similar debris.

Whether or not a body panel is rusted, creation of a uniform, cleanly stripped surface over the entire body panel is a very important part of the usage of this blast cabinet. This uniform surface reduces the amount of random orbital air sander and wire brush work needed prior to acid and zinc treatments done in preparation for primer coats.

To explain... when a highest quality paint job is to be done, primer or paint is not applied directly to the blasted surface. In this high-end type of paint process, blasting does only the 'hard work' of removal of rust, corrosion, debris, etc. It does not serve as paint prep. In other words, blasting is only the final stripping step. It is not the final prep step. After the blasting/stripping operation is complete, random orbital disc sanding removes microscopic debris and any texture left from the media blasting. It is after this disc sanding that repair operations to the panel are performed. This could be repair of dents, cracks, rusted-through areas etc. After repair operations, phosphoric acid cleans the metal surface to the most uncontaminated state possible. Then zinc phosphate is applied. This creates a phosphate coating that bonds to the metal much more aggressively than primer. In addition to its optimal adhesion to the metal surface, the zinc phosphate coat provides corrosion resistance and leaves an ideal, uncontaminated, porous surface for the first epoxy primer coat (ideally chromate) to adhere to. The porous zinc phosphate layer is very absorbent, just as most any other porous substance is. The primer therefore adheres extremely well to the zinc phosphate layer-- which is bonded to the metal surface-- which is sterilized of contaminants by the acid. This process then, as much as is possible 'in the field', duplicates OEM factory processes. Anyone willing to take the time to do long-term comparative test samples can quite easily demonstrate this process to be more durable, corrosion resistant, and longer-lasting than priming (whether epoxy or self-etch) directly on a bare metal surface (whether media blasted or random orbital sanded). Even more importantly from a monetary point of view, it is a fail-safe system. From bare metal on out, each process tends to indicate any adhesion problem immediately and prior to application of the next layer. This, instead of failing later after the paint job is finished. Realize that 'later' could be after tens of thousands of dollars worth of block sanding, color sanding, buffing and polishing has been performed. Applying an ultra expensive, labor intensive paint job that relies on only primer-to-bare-metal adhesion is very, very risky business.

Some important notes about the acid/phosphate method:

A major stumbling block with this system is that it must be done entirely correctly to work as designed. If done incorrectly it will often fail to perform as well as other methods. Unfortunately, it is very seldom done correctly in the field. Therefore the process has an extremely negative reputation among those painters that do not understand how the system works-- whether custom painters or collision industry painters. A primary reason for this situation is that there are no readily available instructions of sufficient detail to facilitate learning this process correctly. This is because the process is much too labor intensive to be used in any type of production repair work in today's world. While it can seem relatively easy to perform the process on a small test sample, taking on an entire car body is a massive operation requiring many man-hours of labor. The process is very much a lost art as far as it being done manually in a small scale, non-manufacturing environment. In contrast, acid cleaning and phosphate conversion coatings-- whether zinc phosphate, iron phosphate or other are very much standard operating procedure in any manufacturing industry that produces quality painted metal items that must continuously withstand an outdoor environment.

A Division of Processes

As was previously stated, this blast cabinet is used mainly for removing rust and other contaminants from steel automotive body panels. It is normally not used for glass bead blasting of engine parts. That work goes to a standard size glass bead blast cabinet. It is usually not used for stripping thick, multiple coats of old car paint nor paint jobs that have been done in modern, durable (flexible) catalyzed paints. Instead of being stripped in the blast cabinet, most such paint is removed by soda blasting, paint remover or other means.

This division of processes is held to because glass beading of engine parts, stripping paint from heavy duty parts, stripping substantial amounts of paint from sheetmetal, and several other common blasting tasks are jobs that are very different from the job of removing rust from sheetmetal body panels. These different tasks are best done with different air pressure, media type, media grit size and even cabinet design. Certainly any combination of these blasting method variables will serve to merely get the job done on either paint, rust or anything else for that matter. However, if the process is to be efficient and leave the parent metal undamaged, it must be tuned to the specific substance being removed and the specific parent material of the part being cleaned. In a pro shop that has to contend with two or more of these tasks, the most efficient situation is to have not just one, but two or more separate blast systems (or appropriate subcontractors), each set up for its own specific task. Otherwise much time is lost to changing media and configurations or to the inefficiency of using a cabinet not well-suited for a given task. That is why this blast cabinet is specifically set up for working with body panels. Would it really make economic sense to take the time to change abrasive, gun, nozzle sizes, etc. of this cabinet in order to glass bead blast an engine part when that part could easily be blasted in a another cabinet of one-fourth the size?

Sandblast Confusion and Hazards

Media blasting is not everyone's first choice for stripping automotive body panels. Some are of the opinion that body panels should be stripped by means other than media blasting of any kind. Others qualify that statement by suggesting that body panels can/should be media blasted but never sand blasted. Both positions are somewhat misinformed in that media blasting can be anything from gentle dry ice blasting or soda blasting to very aggressive blasting or wheel abrading with extremely large, dense, hard abrasives. Surprisingly, so can sandblasting. The effect these processes have on sheetmetal can range from virtually no effect to that of annihilation (literally) of a body panel in a matter of seconds.

The problem with across-the-board elimination of sandblasting as a possible choice for stripping sheetmetal items is that the term 'sandblasting' covers a very wide range of processes. If we assume 'sandblasting' refers to actually blasting with silica sand, the range of effects on a body panel is just as varied as with other types of abrasive media. The 16 grit (0.043") or 30 grit (0.022") silica sand commonly available from local suppliers is only a small part of the full range of available grits. Silica sand is available from many abrasives suppliers in sizes from 4 grit (about 3/16") up to between 220 grit (0.0025") and 320 grit (0.00122"). Beyond that, large specialty suppliers can provide silica as 'flour' and 'gravel' as it is sometimes called. As flour it can be purchased in sizes every bit as fine as the soda used in soda blasting. As gravel it can be as large as good sized crushed rock. The point is a body panel can be blasted with sand and suffer none of the damage associated with 'sandblasting' -- if the appropriate grit size and air pressure is used.

A note about silica dust and other hazardous dust created by other abrasive blasting processes:

As just stated, given the correct method silica sand can be used to successfully strip sheetmetal body panels without damage. Even so... some would suggest that the real question is... should silica sand ever be used to blast anything?

It is quite commonly known that prolonged inhalation of very fine silica dust causes silicosis. This is a very serious lung disease that can lead to lung cancer. Thus silica dust is classified as a carcinogen by many health organizations. Many abrasive media suppliers and government agencies recommend against any use of silica sand as a blast media. Those same sources suggest various alternative types of abrasive media. Not surprisingly, those alternatives are often much more expensive than silica sand. Unfortunately... often little or no safety warning is emphasized or even given for the suggested alternative media. That makes it easy for most users to assume the alternative media is safe, or at least more safe. That is a problem.

Coal slag based abrasive (trade names often contain the term 'Black') is one of the most, if not the most, widely used of all substitutes for silica sand. The fact that it is produced from coal slag is usually down played and therefore unknown by the end-user. It is usually marketed (at a higher price) as a safe, silica free alternative to 'silicosis-causing silica sand abrasive'. Yet the published papers of numerous respiratory hazard studies state that coal slag abrasive media dust causes greater harm in a shorter time to the respiratory system than does silica dust. So, while it does not cause silicosis, it is more rapidly hazardous to human health. That is unfortunate of and by itself, but add the following thought. Most people are aware that silica sand dust is hazardous and take measures to protect themselves while near it. At the very least they are conscious of the fact that they are working with a hazardous substance. On the other hand, an uninformed user of coal slag abrasive buys it on the assumption it is safe or at least more safe than silica. He is entirely willing to pay more for the alternative than for silica sand based on the idea that he is investing in his health. Since he feels he has spent money in order to avoid health risks, he proceeds as if the abrasive dust is not a hazardous substance. The thinking is... "This alternative abrasive media contains little or no silica. Therefore it can't cause silicosis. Therefore I don't need to worry." He may not even feel the need to take any precautions whatsoever. The net result is that while he thinks he has invested money to preserve his health, the reality is he is at greater risk. He is at greater risk both because of the dust being more immediately harmful and his attitude being more relaxed.

None of this is meant to be either a condemnation of coal slag abrasives or a defense of silica sand abrasives. They are both hazardous to human health. All abrasive dust is hazardous. Some is more hazardous. Some is less hazardous. It is all different only by degree. The situation is the same as prolonged exposure to different types of paint fumes. While one type may 'kill ya dead' faster, the others can still 'kill ya dead' slower. Before doing any blasting of any kind it is only reasonable to research the subject of media blasting health hazards instead of relying on the claims and warnings of abrasive media suppliers. It is not difficult to locate published information on the subject on the internet and elsewhere. There are many occupational health safety research reports and many respiratory disease studies about silica, coal slag, soda, garnet and other abrasive media types. By researching this matter an informed decision can be made about what abrasive(s) best fits someone's performance/cost/safety requirements. The important thing to remember is that no abrasive media dust is entirely harmless to the human respiratory system. What's more, even if a relatively 'safe' abrasive could be devised and produced, debris from whatever is being stripped (paint, rust, mill slag or anything of the kind) is itself a hazardous substance that should not be inhaled. The key point is-- all such dust in the high concentration levels found in media blasting is hazardous to one degree or another.

Media Blasting Damage of Sheetmetal Panels

When media blasting metal castings, heavy fabricated metal structures, or any such more or less indestructible items, if the process is not proceeding at a reasonable rate changes can be made to speed the work up. Coarser grit media, heavier media, sharper media, higher air pressure at the blast nozzle, greater volume of flow at the blast nozzle, or any other more aggressive means of blasting all serve to speed up the stripping process. This is generally true no matter whether paint, rust or some other substance is being removed. In the case of those 'indestructible' parts, this more aggressive blasting is often acceptable because there is little chance of any significant damage to the item. Body panels made of sheetmetal are an entirely different matter though.

Aggressive or even normal blasting (normal for heavy metal parts that is) can cause damage to sheetmetal in a number of ways. Warpage, a too-coarse surface texture, and work-hardening of the metal are very well-known problems. Warpage and coarse texture are visually apparent... as are the problems they create. Obviously, warpage will lead to a very wavy paint job unless it is corrected. A coarse texture will make paint prep work more time consuming. The best solution is to sand the coarse texture off with a random orbital sander. Work-hardening though, is not a visually observable problem. When it occurs, what problems it causes, and how to avoid it are much less obvious than texture and warpage situations.

To quantify how much work-hardening is 'a lot'... most everyone who has done any great amount of body panel dent repair has run across at least a few instances of severely work-hardened metal. This usually is in instances of repairing damage that has occurred in the same location as a previously done extensive repair. If the previous repair person used a lot of on-dolly hammer work, extensive water-quenched heat shrinking, and whatever other deadly deed to achieve the repair, that localized area is often work-hardened to the point of being extremely difficult to repair again (i.e. has much higher yield strength). Enough blasting in a localized area using a very aggressive blasting media/method can achieve similar levels of work-hardening.

Work-hardening, that is to say work-hardening that is severe enough to cause problems, occurs to sheetmetal when the blasting method is too aggressive. As the blasting method is made less aggressive the degree of work-hardening is reduced until it becomes of no real consequence. Aggressive blasting acts much like a shot-peening operation. It serves to increase the yield strength of the metal. Technically, it is affecting only that metal at the surface or near the surface of the item being blasted. However, in light of the fact that body metal is quite thin and in light of the fact that both sides of the panel are often blasted-- the net effect is that a significant percentage of the thickness of the metal is being hardened. The effect can be substantial with sheetmetal whereas it is often minimal with heavy chassis parts of large cross-section. To explain... consider two test strips. One is a length of flat bar. The other is a length of sheetmetal. The overall percentage increase in yield strength of the test strip that could occur as a result of media blasting a 1/2" thick x 6" wide length of flat bar would be extremely small. On the other hand, the overall percentage increase in yield strength of the test strip that could occur as a result of media blasting a 6 inch wide length of 20 gauge sheetmetal can easily be great enough to make dent repair a real chore. The flat bar example contains a high percentage of metal deep within the cross-section that is unaffected by the blasting. Given an equal amount of blasting, the thick flat bar simply does not gain as much (percentage) strength as the sheetmetal. That is because a large percentage of the metal is unaffected. On the other hand, since the sheetmetal strip is so thin, a much greater percentage of the metal it contains is affected by the blasting. Therefore, the sheetmetal part-- the test strip-- realizes a greater percentage gain in yield strength than the flat bar. What this means is that while aggressive blasting generally has little if any net effect on the yield strength of a heavy structural part, it has a dramatic effect on the yield strength of light-gauge sheetmetal.

In theory a uniformly work-hardened (relative to its original state) steel body panel could be a good thing as long as it was not somehow hardened to the point of being overly susceptible to cracking at flanges and brackets. It could be a good thing, that is, if the only criteria was greater resistance to being bent, dented or otherwise damaged. The work-hardened panel would have higher yield strength than it did before it was work hardened. Therefore it would be more resistant to minor dents and dings. Unfortunately it would also be more resistant to repair of those dents. The predominant opinion is that since the average old car/custom-built car/specialty car's mild steel body panels of 18, 19, or 20 gauge standard cold roll mild steel are amply strong as is... and since any damage and consequent repair of an existing body panel will induce some amount of work-hardening on its own... that it is therefore most reasonable to not introduce any work-hardening unnecessarily. Save any workability left after the initial making of the panel (whether by OEM die-stamping or custom building) in reserve for those occasions when it is needed-- such as collision repair, etc. This all assumes the discussion is limited to body panels of mild steel. Body panels of higher alloy steel such as HSLA, stainless, etc. or very low-carbon content soft steels might be subject to slightly different considerations.

A note on quantifying work-hardening:

It might be assumed that short of having access to some sort of elaborate metallurgical testing laboratory, it is impossible to determine how much work-hardening is being induced in a sheetmetal body panel by a given media blasting process. This is not the case. Although such processes are more involved than merely bending a couple strips of metal to see if a difference can be felt, sheetmetal test strips can be comparative tested without special equipment. Comparative testing can identify small increases in yield strength. It does not matter whether that yield strength increase occurs because of media blasting, hammer and dolly work or even English wheel shaping. Testing of sheetmetal sample strips can show differences. Such strip test methods don't provide numeric values but that is of no real concern in this situation. Comparative difference is what matters here. The real need is not to acquire a numeric value. The real need is to be able to answer the question, "Is sample 'A' work-hardened more than sample 'B'?" Or perhaps more to the point regarding media blasting, "If abrasive type 'X' in grit 'Y' is being used and sample tests are done at progressively lower blaster gun air pressure settings, at what pressure does work-hardening fall to a level that is deemed by the metal worker to be of no concern?"

There are also some less well-known problems created by overly aggressive blasting of body panels. One is that rust in the bottom of a rust pit in the surface of the sheetmetal can be somewhat hidden by the gouging, pitting action of coarse, high velocity abrasive. The edge of the perimeter of the pit can be slightly peened down into the cavity of the pit thereby covering up some amount of rust. Visually this all becomes part of the deeply textured surface and abrasive contamination (explained below) left by aggressive blasting in general.

A related problem that can hide rust and rust pits that are still present in the metal is that of abrasive contamination of the sheetmetal surface. This occurs when certain highly friable abrasives are used. Probably the most extreme example of this is sandblasting. A surface that has been very recently sand blasted (meaning blasted with silica sand) is considered by some to be the ultimate in a clean, ready-to-paint surface. It is not that at all. What it is... is a filthy, dirty surface as far as what is desired for paint prep. It is an extremely poor surface for priming or painting. Even after being blown 'clean' by compressed air to remove visible media dust, the whitish-gray surface that is assumed to be the appearance of ultra-clean bare metal is in fact the appearance of microscopically-small, fractured silica particles embedded in the metal surface. Substantial rust can be hidden under the embedded silica. These embedded silica abrasive particles are not well-attached to the metal's surface. Consequently, if rust pits, overly rough surface texture and/or silica particles are primed and painted over, problems eventually crop up. The result can be adhesion failures, continued rust corrosion and even reduction of DOI (Distinctness of Image) and gloss in the paint system topcoat. The latter is very common and often mistakenly assumed to be a paint topcoat solvent-related gloss die-back issue.

Finding hidden rust:

This situation of surface texture and/or embedded abrasive covering over rust can often be verified with a random orbital sander using 100 to 120 grit paper. After a heavily rusted sheetmetal surface is blasted with silica sand (or one of the other more friable abrasives) using either normal or greater than normal air pressures and abrasive grit size, sand the surface with the random orbital sander. Unless a very thorough blasting job was done, the orbital sanding will likely reveal some amount of rust missed and covered up by the abrasive contamination.

However, even though embedding of small particles is the case with sand and some other media, do not jump to the opposite extreme and assume that all media types will embed themselves into the sheetmetal surface to the point of contamination. It just is not so-- even though some anti-blasting commentaries claim it is so.

Extremes of Damage/No Damage

To insure that body panels are not damaged by the blasting process, the most obvious solution is to use a less aggressive procedure. Lower air pressure at the blast nozzle, finer grit media, lighter media, or otherwise less aggressive means of blasting all serve to help prevent damage to sheetmetal panels. Unfortunately many of these factors drastically slow down the rate at which the stripping occurs.

At this point --meaning the point at which damage to sheetmetal becomes an issue-- there is a real parting of the ways between what is the most appropriate blasting process for a 'fragile' sheetmetal part versus an 'indestructible' chassis part. Even more importantly there is a real parting of the ways between what is most appropriate for stripping paint from sheetmetal versus stripping rust from sheetmetal. Getting back some of the speed that is lost by the need to take measures to reduce sheetmetal damage is more readily done if rust is the substance being stripped. It is more difficult if paint is being stripped. Another way to say this would be as follows. Trying to increase the rate at which a blasting process strips either rust or paint from sheetmetal is very much at cross-purposes to trying to prevent the blasting process from damaging that sheetmetal. It is, however, at less of a cross-purpose if rust is being stripped than if paint is being stripped.

Consider the task of stripping a urethane paint job from a car fender. Sixteen grit silica sand in a blaster at 100+ psi air pressure would rip this paint off at a relatively fast rate. It would also leave an extremely rough texture to the metal surface. Unless extreme care was used and the blast stream was kept from being aimed entirely perpendicular to the surface, warpage of the fender would result. Finally, no matter how shallow an angle the blast stream was restricted to, the metal would be work-hardened to some degree. In an attempt to reduce this damage to the metal, blasting might be attempted with say 120 grit sand and much lower air pressure. Unfortunately the lower velocity, smaller grit abrasive has less effect on the paint. Paint, even fully cured paint, is relatively soft and rubbery compared to rust or metal. The combination of finer abrasive grit and reduced pressure would mean that many of the abrasive particles would merely bounce off the paint layer instead of aggressively cutting into it as the 16 grit high pressure setup would. This is because the finer, smaller particles have less mass than the 16 grit particles. They are also traveling at less velocity as a result of the lowered air pressure. This means they are striking the paint with much less momentum. It is true that more abrasive particles per second are at work than with 16 grit abrasive, but not enough more. As abrasive grit size and air pressure are both reduced, at some point little or no damage would be done to the sheetmetal. Unfortunately, little if any paint stripping would occur either.

A way to recover from the reduced stripping speed without significantly increasing damage to the metal surface would be to capitalize on the just mentioned concept of providing dramatically more abrasive particles per second. In other words, greatly increase the volume of flow of the blast gun while using much less aggressive blast media. To accomplish this to any meaningful degree when stripping paint the compressor, blast gun, hoses and related components would all need to be of larger capacity. More abrasive media would be required since it would be used at a faster rate. One of the most extreme examples of this concept is of course soda blasting. By using an abrasive that is dramatically finer, lighter and softer than the 16 grit silica sand in the above example of paint stripping, damage to the metal is very greatly reduced or even eliminated. In soda blasting, the smaller momentum per particle and therefore slower stripping speed is overcome by directing a huge volume of the relatively harmless abrasive particles at the paint by use of much higher capacity (and much more expensive) equipment. Also, soda has extreme friability (tendency to fracture). Therefore, each tiny particle of soda fractures on impact with the paint coating instead of bouncing away as some less friable abrasive would. This extreme friability allows all of the energy of the moving particle to be delivered, in theory at least, to the paint surface.

Blasting with soda causes dramatically less damage to a car's sheetmetal, glass, chrome trim and rubber than blasting with silicon carbide, aluminum oxide, silica sand or similar media. Unfortunately it also does much less 'damage' to the rust on the sheetmetal panel. Meaning, soda blasting is not the best choice for rust removal. Other types of abrasive offer far better performance for that task.

A cautionary note:

When having a soda blast facility strip paint from a valuable car, make certain of the facility's technique for dealing with rust. To speed up the rust stripping process, alternate methods are sometimes used. One such alternative is to blast the rust with a mixture of soda and aluminum oxide. More commonly and sometimes without even informing the customer, some soda blast companies simply sandblast rusted areas with a common, everyday pressure pot sandblaster with coarse grit silica sand and 90 to 125 psi air pressure prior to the soda blast. Thus, the very thing the customer is shelling out money in order to avoid is, unknown to him, being done to his very valuable sheetmetal. Rusted areas of sheetmetal are being severely work-hardened and coarsely textured if not warped. As a result of such behind-the-scenes practices, some hobbyists mistakenly believe soda blasting is an extremely fast and efficient rust removal process.

So... it can be seen that some types of aggressive media blasting methods rapidly remove paint and rust but simultaneously damage the sheetmetal itself. At the other extreme, some very mild types of media blasting are capable of removing paint or even very heavy layers of paint with virtually no sheetmetal damage but are slow or ineffective at removing rust. For efficient rust removal what is needed is something between those two extremes-- a 'happy medium'. Even more importantly what is needed is to do away with the requirement to remove paint. Instead, the process needs to be specifically aimed at removal of rust. This is not to say that paint should not be removed by blasting. It is to say that it makes little sense to try to remove both rust and paint (without causing sheetmetal damage) with just one single blast media/method. Using one single blast media/method-- particularly when the requirement of not damaging the sheetmetal is thrown into the mix-- is an inefficient process.

Preventing Media Blasting Damage of Sheetmetal Panels

As mentioned in the previous section... when a normal blasting process is used to strip paint, often the media and process used is relatively aggressive. Coarser grit abrasive will usually provide faster paint removal than the same abrasive type in finer grit at the same air pressure. That is not usually true for blast stripping of rust though. This section explains why.

Some points to keep in mind while reading this section:

The phrase 'normal blasting process' refers to simple, traditional equipment having average air pressure and flow capacity that delivers common, everyday abrasives such as sand, garnet, aluminum oxide, etc. This, as opposed to high volume and/or high pressure or otherwise highly specialized processes such as soda blasters, ice or dry ice blasting, and high capacity industrial plastic media blasting systems. As was previously stated, soda and some other blast processes overcome the less aggressive nature of their smaller, lighter media by delivering a huge volume of particles. This very high volume is not possible using standard, 'normal blasting process' equipment. It is very important to keep this distinction in mind in order to understand what might otherwise appear to be contradictory statements about grit size, etc. These two different types of equipment are just that-- two different types of equipment.

This article and especially this section of this article refers to those instances in which blasting must be made less aggressive in order to prevent damage to sheetmetal. It does not refer to those instances when high air pressure and/or coarse abrasive can be used without regard to any negative effects they may have on the item being blasted.

Rust and paint are very different substances as far as requirements for optimal blast stripping. Rust is a brittle substance that can be easily fractured into pieces. It is often embedded into the metal itself. Paint and primer by comparison is strictly a surface coating. It is much more resilient and flexible than rust. To carry the contrast to an extreme, paint could be thought of as a rubber-like layer that must be torn away from the surface by the abrasive. Rust is a weak, brittle, solid that the abrasive (if it is hard enough) can instantly fracture into much smaller particles. However, to do so the abrasive must be able to get down into the rust pits below the metal's primary surface. Also consider that some paints are more resilient and flexible than others. An aged, hard, brittle lacquer paint job can be more easily blast stripped with normal equipment than can a very flexible and durable urethane paint job. This in much the same way that a brittle paint on a car's rocker panel is more susceptible to rock chips and other impact damage while the car is being driven than is a flexible, durable urethane paint.

Again, repeating what was just mentioned, at the moderate air pressure and air volume levels of a normal blast system, a very fine grit abrasive can mean slow progress when stripping paint. Compared to a larger grit abrasive, the small grit has lower momentum (unless air pressure is increased in an attempt to compensate) and tends to bounce off of paint instead of penetrating into it. A larger grit tears into the paint surface much more aggressively. So, at the same air pressure, even though there are fewer numbers of abrasive edges and corners present when using a coarser grit, that larger, heavier, higher momentum particle is better able to penetrate the rubber-like surface of the paint. However, when the goal is to remove rust --and the blasting process must be less aggressive in order to avoid damage to sheetmetal-- abrasive grit size has a more complex effect on performance. This is true no matter what choices are made regarding the cost/performance tradeoffs that various abrasives provide through differences of density, sharpness, friability, durability, etc. Indeed, those cost/performance tradeoffs are still important decisions that need to be made. Nonetheless, particularly at low air pressure levels, the wrong abrasive grit size can really slow rust stripping performance down no matter how those other choices are decided upon. Explanation of this is not exactly a simple task...

In the quest to avoid damage to sheetmetal, the simplest means of making a rust-removal blasting process less aggressive is to lower the air pressure at the blaster gun. This requires no time or investment. Anybody can turn the knob on the pressure regulator counter-clockwise. The next easiest option is to reduce the abrasive particle size. This requires the time and expense of a change of abrasive. In most cases a finer grit version of whatever media type is being used can be purchased for the same or nearly the same price as the coarser grit. A combination of these two options-- reduced pressure and reduced grit size-- is usually all that is needed to avoid damage to sheetmetal body panels.

The first option, reduction of the blaster air pressure, decreases the velocity of the abrasive particles. For any given abrasive particle weight, a reduction in velocity causes the particle to hit the sheetmetal surface with less momentum. Particles with less momentum do less damage to the sheetmetal. A comparison is that of hitting a sheetmetal surface with a hammer of given weight. If the hammer is swung one time as fast as possible the sheetmetal would be severely dented. But... the very same hammer could also be swung one time very slowly, slowly enough that the sheetmetal would not be dented at all. Unfortunately, when blasting rust from a panel, decreasing the blast air pressure also reduces the amount of work being done. That is, a decrease in pressure reduces the speed at which the blaster removes rust. However, if the volume of flow of air and abrasive is increased in order to compensate for the lower pressure, the rate of rust removal will be increased again by some amount. To explain: Assume a blaster gun of given size is normally supplied with 100 psi of air pressure measured at the gun. If the same gun with the same size air jet and nozzle is supplied with only 50 psi of air pressure (at the gun) it will provide a smaller, slower abrasive stream-- meaning it will supply fewer abrasive particles per second and those particles will be moving at a lower velocity. Now assume a larger flow capacity gun, or at least one with larger air jet/ceramic nozzle combination, is supplied with 50 psi air pressure (at the gun). At that 50 psi it will provide a larger abrasive stream than the smaller capacity gun operating at 50 psi. The abrasive velocity will be the same but the larger flow capacity will supply more abrasive particles per second than the smaller capacity gun.

The other option, reduction of abrasive grit size, serves to decrease not only the size of the abrasive particle but also its weight. Reducing the weight of the abrasive particle reduces its momentum at any given blaster gun air pressure setting. Particles with less momentum do less damage to the sheetmetal. Again a good comparison is to equate this to the situation of striking sheetmetal with a hammer. Consider two hammers. One is very heavy and the other is very light. If both hammers are swung at the sheetmetal at the same speed (one hammer in each hand perhaps) the heavier one will cause more damage. Everyone is familiar with the old saying, "Don't force it, get a bigger hammer."

A note about size:

This suggestion of smaller size may seem to be the same as soda blasting wherein the abrasive is so small and light as to not be aggressive enough to be entirely effective as a rust remover. There are distinct differences though. Soda abrasive is extremely small, light, soft and friable. The reference here however is to more traditional abrasives that, while less aggressive in smaller grit sizes, are still much more aggressive than soda blasting. They are in the 'happy medium' range needed for fast rust removal. Indeed, well short of moving down to the size of soda there is a point of diminishing return. To effectively strip rust, an abrasive particle must be large enough and dense enough... that each particle has enough mass... that it has enough momentum at the velocity at which the compressed air exiting the gun accelerates it... that it strikes the rusted surface with enough force... that it actually accomplishes something. Even when using more traditional abrasive media type, abrasive grit size can be too small for efficient rust removal.

It might seem that using smaller size abrasive grit is a necessary evil that is begrudgingly resorted to as a means of reducing damage to sheetmetal. It might also seem that since a smaller abrasive particle does less damage to the sheetmetal, it may also do less 'damage' to the rust. That is to say that if it has less momentum it may be doing less rust stripping. That may be true as far as the net result provided by each individual particle. However, as is the case with reduction of air pressure settings, volume is at play here as well. Since the abrasive particles are smaller, more of them are at work for any given blaster gun size/air pressure setting. The smaller the particles are, the more of them strike the sheetmetal per second.

Aside from just the matter of there being a greater number of particles per second leaving the blaster gun, there are other reasons why finer abrasive has the potential for removing rust faster than coarse abrasive at the same air pressure. The smaller an abrasive particle is, the deeper into a rust pit it can travel. The smaller the particles are the more of them that can fit in a rust pit at one time. The more particles in a rust pit at any one time, the more sharp edges and corners there are to contact the rusted surface. This is true not only of large, visible rust pits but also 'surface rust' on a relatively undamaged surface. All rust of any intensity is essentially a 'pit'. Its only a matter of how far the rust has progressed into specific clusters that determines whether it is referred to as 'surface rust' or 'rust pits' or 'cancer'.

It is helpful and perhaps even reassuring to note that a similar and more commonly known relationship between abrasive grit size and stripping speed exists with some disc sanding operations.

When a random orbital sander is being used it is assumed by some that coarser grit discs always result in faster cutting. When sanding some substances that is true. In other situations it is not true. An example of the latter is the sanding of what is essentially bare sheetmetal to remove problems such as light surface rust, debris of one sort or another, or weld seam HAZ discoloration. For that type of cleanup sanding job, 120 to 150 grit discs often work faster and last longer than either 80 grit and coarser or 220 grit and finer. The 220 and finer grit disc is just too fine to cut very fast and typically clogs up with residue. The 80 grit disc does not have nearly as many abrasive particles per square inch as the 120 to 150 grit disc. While the individual particles of the 80 grit disc will cut deeper and more aggressively than the 120 to 150 grit particles there are fewer of them. Since they are fewer in number they require more passes and they grow dull faster. So, for this specific type of sanding, the greater number of abrasive corners and edges on a 120 to 150 grit disc will often result in a net increase in speed and disc longevity over the more aggressive but fewer number of cutting edges available on an 80 grit or coarser disc. This assumes the sheetmetal surface texture is reasonably good. Extremely coarse texture will of course require coarser sanding disc grits. For example 16 to 24 grit grinder marks or extra-coarse blast texture is not going to be readily removed with a 120 grit sanding operation.

Again, this 'happy medium' situation is not always true for sanding all substances, particularly soft, porous substances. For example, an 80 grit disc will remove body filler much faster than a 150 grit disc. A 40 grit disc will often remove filler much faster than an 80 grit disc and so on. Although... many primers and primer surfacers have a similar optimal, not too coarse-- not too fine 'sandability zone' wherein a given sandpaper grit can be too coarse for fastest cutting, especially while block sanding by hand. A slightly finer grit may cut faster while a much finer or much coarser grit will cut slower, and in some instances less flat.

The ability of an abrasive particle to get into and abrade the rust out of a pit is key. Because smaller particles are better able to get to the bottom of a pit there is much less need to concentrate the blaster gun's stream of abrasive on one location for any extra length of time or to make great numbers of passes over any one area. On the other hand, when large abrasive grit size is used on rust-pitted sheetmetal there is a need to do just that-- to either hold the blast stream in one place longer or to make multiple passes over each region in order to get all of the rust. So not only do larger blast particles tend to cause more damage, by their very nature they can require more passes and/or longer blast time. The longer blast time compounds the problem. This is typically what leads to warpage of panels by inexperienced or unconcerned blaster operators. And... it certainly is how panels become highly work-hardened and rough-textured.

The concept of the 'happy medium' is important. If a test is done in which smaller and smaller abrasive grit size is used, at some point the rust stripping process would begin to be slower and slower until it became unrealistically slow for automotive panel work. It is true that abrasive grit size can be too small. Consider the opposite extreme though. If abrasive grit size was on the order of 1/4 inch in diameter it would not matter how long the blaster gun was aimed at a heavily rust-pitted area. It could be held on the rust pits until a hole was ripped into the sheetmetal panel. The rust would still be in the bottom of the rust pits. The abrasive particle is simply too large to abrade the rust out of the pit. So... someplace between these two extremes, at a given air pressure, there exists a particle size that will remove rust faster than either a larger size or a smaller size. Of course the size of the rust pit comes into play here and not all rust pits are the same size. But the point is... bigger is not always better. There is an optimal grit size range.

As far as damage to a sheetmetal panel, it could be argued that using smaller abrasive grit does not reduce net total tendency to warp or work-harden. The argument would be... When using larger grit abrasive, even though fewer particles per second are striking the panel, the fact is each individual particle has more momentum. When using smaller grit abrasive, even though each individual particle has less momentum, more particles per second are striking the panel. Therefore the combined net detrimental effect of all of the more numerous smaller particles is just as great as that of the heavier but fewer in number larger grit's particles. Testing can show the reality is it does not work quite like that. For any given amount of rust stripped, smaller (and therefore lighter) particles produce less total work-hardening. And... real-world testing can always verify whether a great amount or a minimal amount of work-hardening is occurring.

A similar argument could be made regarding comparison of a blaster gun having a higher volume of flow with one of lower volume of flow-- both operating with the same air pressure and abrasive grit size. The reality is that for the most part the higher flow gun is simply providing a larger pattern to work with. The end result is, the stripping process is simply occurring over a larger area at one time. It is not appreciably hardening the metal at a greater net rate.

Although not exactly the same, consider a hammer analogy again. Only a few seconds of very aggressive on-dolly hammer and dolly work in a small area can cause substantial stretching (equates to warpage) and work-hardening. At the other extreme, a panel could be struck on-dolly with a very small, light hammer at such a very slow impact velocity that virtually nothing would occur. The hammer would be applying such a small force per unit area to the panel that it is below the threshold of causing any noticeable, immediate effect. The number of times the panel is hit does not matter in this case-- short of continuing on for many millions of cycles with fatigue test apparatus in order to generate some long-term metallurgical change.

These are complex issues that involve difference only by degree. The overall concept is relatively simple though. Using a relatively aggressive media type, select the smallest abrasive grit size that seems to strip rust well. Use the lowest air pressure that seems to give good progress without undue negative effect. Use a blaster gun with the largest volume of flow that the air compressor can support at the blaster gun air pressure setting being used. These three factors serve to optimize the rate at which the rust is stripped. Optimizing the rate at which rust is stripped in turn reduces the need to either aim the blaster gun at any one place for a long period of time or to make multiple passes over an area. Again, real-world testing shows that this methodology does indeed reduce work-hardening and can entirely eliminate warpage and rough surface texture.

Abrasive Media Properties

Many of the physical properties of abrasive media affect its blasting performance on sheetmetal items. Some of the most relevant properties are listed here. A key point to realize is that most of these characteristics are interrelated. It is not really possible to neatly divide up all aspects of abrasive performance into a list categorized by individual physical property, but a few relevant points are made below. This is by no means the sum total of all information about abrasive characteristics. It is just a starting point that hopefully leads one into more detailed research specific to his own blasting job requirements.

Grit Size

Abrasive grit size is obviously important since it can have a tremendous effect on the aggressiveness of the process and the amount of unwanted damage done or not done to sheetmetal. However, since most of the 'normal blasting process' abrasive types are available in a vast range of sizes, grit size is often not a necessary part of comparing one abrasive's performance with that of another. That is to say, most standard abrasives can be had in whatever grit is needed to match up with any other abrasive. So... grit size can often be factored out of a performance comparison between two types of media. (Soda, etc. excepted.) Therefore, in this section of the article, grit size is mostly discussed as it relates to other abrasive characteristic. (See Density and Weight below.)

As far as the more common abrasive media, grit size can range from 4 to over 300. More typically, available sizes range from 16 to 120. In this range 16 is usually considered appropriate for heavy duty, coarse work. Grits of 20, 30, and 40 are most common. Grits of 80 to 120 are considered very fine. Some specialty abrasives such as soda are so extremely fine they are measured in microns instead of grit numbers. Sometimes spherical shaped abrasive is graded by the actual particle measurement in inches instead of grit size numbers. Glass bead size is classified by many different methods.

Abrasive type and grit size availability is very limited in many local markets. One alternative is to buy from specialist suppliers catering to the do-it-yourself market. This type of supplier often has only one or two grit size choices for a given abrasive type though. This type of supplier usually ships from only one location. That can mean freight charges for shipment of the abrasive over thousands of miles. That can be very expensive. Another choice is to buy from major regional industrial suppliers. Most have quite competitive prices and very extensive selections of abrasive type and grit sizes in stock. For those not near such a supplier, a search of the telephone directory of the closest population center of any size and also the closest major metropolitan/industrial area usually finds several such suppliers. There is often a trade-off of shipping cost versus abrasive cost per pound. Abrasive cost may be less from a supplier in the nearest major metropolitan/industrial area but shipping distance might be greater. On the other hand, shipping distance from the nearest city/town may be less but abrasive cost per pound may be substantially higher.

Sharpness

Comparative sharpness ratings of different kinds of abrasive particles is difficult information to come by. In instances when that is not available, sharpness can be somewhat equated to abrasive particle shape. Abrasive can be any shape from highly angular with infinite numbers of sharp edges like soda media to entirely smooth and spherical with no sharp edges like round steel shot media. Common sense tells a lot. For example, silicon carbide has razor sharp edges in comparison to round steel shot abrasive. Physical comparison of samples can be reasonably accurate. For example some 'play sand' is less sharp than aggressive sandblast silica sand.

Sharpness of the abrasive influences the type of finish applied to the substrate (metal) surface. Obviously a sharp particle would tend to create distinct peaks and crevices while a 'dull' particle would create a dimpled texture.

Shape

The shape of abrasive media particles affects how other properties of the abrasive particles manifest themselves. Consider two abrasives of similar grit size, friability, density and hardness. Assume the hardness and density levels are quite high. Say that one of the abrasives is smooth and spherical (or at least rounded) in shape but the other is multi-edged and angular. Even if these two abrasives had identical properties other than shape, they would have very different effects on a sheetmetal surface. The multi-edge abrasive would etch a very coarse, pitted texture into the surface. The spherical abrasive would create a smooth, dimpled surface texture.

Friability

Friability is not a measure of how well a substance can be cooked in a cast iron skillet! A dictionary definition of the term 'friable' is usually something like... "easily reduced to tiny particles". The term 'friability' as it pertains to blast media is simply a reference to how easily the particles break down into smaller particles. Some mistake an abrasive's hardness with its friability. Friability is not the same as hardness. As an example, silica sand is quite hard but it is also very easily pulverized into dust during the blasting process. Steel grit or shot is similar in hardness to silica sand but it is much less susceptible to being immediately fractured into dust like silica.

Highly friable abrasives are normally considered to be 'one-pass' abrasives. Meaning... they are usually used only one time since they are so friable that they fracture into much smaller particles on only one impact with the workpiece. This type of abrasive is usually low in cost as far as a per pound cost. For that reason, it is often used for non-recovery blasting. That is, it is used for blasting with a traditional outdoor pressure-pot or siphon-pot blaster instead of a cabinet-type blaster. Friable abrasives include silica sand, garnet, soda, etc. On the other hand, abrasives with low friability do not fracture nearly so much and are therefore well-suited for blast cabinet recovery type blasting. Instead of fracturing, this type of abrasive tends to 'wear out' as it is cycled through the blast cabinet over and over. The sharp edges become less sharp in much the same way a grinder disc gradual wears dull. Low-friability (or at least low-er friability) abrasives include glass beads, plastic media, silicon carbide, aluminum oxide, metallic grit/shot, etc. It is important to realize that in most cases friability is not an either/or issue but one of degree. Some few abrasives are indeed at the far-end extremes of being either very highly resistant to fracturing into smaller particles or prone to immediately being pulverized into airborne dust. Most however are somewhere between those two extremes.

There are at least two more concepts that are important as far as the relative friability of different blast media abrasives. 1) The abrasive's tendency to fracture is somewhat dependent on air pressure settings even if those settings are within the range of what is considered normal. 2) The tendency to fracture may be seen as an advantage or a disadvantage depending on the nature of the blasting operation being used.

To explain... if a predominantly low-friability rated abrasive is being used in a recovery-type blasting process, too-high air pressure may cause it to become what is essentially a friable abrasive. That is to say, it can be broken down into a size finer than what is desired instead of being gradually worn dull. In extreme cases, too much air pressure can quickly turn expensive and what is normally long-lasting media into not-so-useful dust. At the other end of the spectrum, very high friability can be an advantage, at least in theory. As alluded to previously, in soda blast stripping of paint part of the concept is maximum use of energy. If high enough particle velocity is attained, the particles can be entirely fractured on a single impact. As each particle strikes the paint surface, the more sub-particles it fractures into the more sharp edges and points there are to abrade the paint. Ideally there are no large particles of soda media that bounce off the paint. Thus, almost all available energy is directed at the paint surface in an effort to abrade it away-- in theory. Compare this with stripping paint with silica sand or similar abrasive. Many of the particles simply rebound in one piece at high velocity off the relatively soft paint surface. The particle then strikes some other object, expending its energy there instead of on the painted surface. Or... it continues off into the air until its velocity falls off to zero and no energy is left. In either case, energy is lost to something other than the paint surface being stripped.

Even so, some of these highly friable abrasives are usable more than once. For instance when silica sand and similar abrasive fractures, it fractures into entirely new but smaller and very sharp-edged sub-particles. These sub particles, when re-used, fracture into even smaller particles. Often though the fracturing process is less extreme as the particle becomes smaller. What this means is that 20, 30 or 40 grit silica sand immediately fractures into 80 to 120 grit silica sand and is useful for at least several passes as a still-sharp but progressively finer grit abrasive. This can be an inexpensive but very dusty, dirty way to remove rust from sheetmetal panels.

Density and Weight

The heavier an abrasive particle is, the more momentum it has at any given blaster gun air pressure. The more momentum present, the more aggressive the blast process can be. It is slightly an oversimplification to say that heavy abrasives are used when maximum effect is desired on the substance being removed (rust, paint, etc,) and light abrasives are used when minimum effect is wanted on the surface underneath (aluminum, glass, etc.). It is an oversimplification in that there are exceptions, but it also is very much the norm.

As far as its effect on momentum, abrasive weight can be heavier or lighter as a function of either the size of the particle or the density of the abrasive substance. A large particle of low-density abrasive material could weigh the same as a small particle of high-density abrasive material. These two different particles could have the same momentum given the same blast gun air pressure. They would have different performance characteristics though because of the size difference. Very likely they would have many other performance differences as well. For this reason, particle weight of and by itself is not a very meaningful indicator of performance. Therefore abrasive is often rated by the size (grit rating) of the particle and the density of the abrasive substance instead of by the weight of individual particles.

Weight, or more correctly, density, is a specification that is easily found for most any abrasive. Manufacturers and dealers usually can supply this information in several forms. For better or worse, the most commonly used method of comparing the relative densities of different abrasive media is by specific gravity numbers. Unfortunately, specific gravity is not necessarily an everyday term for all normal humans. Specific gravity is a numeric value that compares the density of something to the density of a standard, usually water. It can be relatively easily determined by real-world measurement combined with a few simple computations. However, it is usually expressed as a metric system oriented number. That is because the metric system was developed using water as the standard for weight (mass) in defining it's basic unit of weight, the 'gram'. The English system, being many centuries old, was largely based on marketing of agricultural goods. Barleycorn was used as the standard for weight to define the English system's basic unit of weight, the 'grain'. It is infinitely easier to do real-world determinations of the specific gravity and density of various substances by using water and units in g/cm3 than it is by using barley and units in lb/ft3 !

Mathematically converting metric-based specific gravity numbers to references of weight or density in pounds-foot (g/cm3 to lb/ft3) after the fact of determining a substance's specific gravity can be done. It is not difficult but it is a nuisance. It is much easier to just use the raw specific gravity numeric values provided in most all published data from abrasive manufacturers as a way to mentally compare the weight (density) of different abrasives. Looking up values of a few common substances provides enough reference to go on. The following list shows the specific gravity values for a few common items that everyone is familiar with.

These values can be used as reference points when considering different abrasives. For example, when comparing one abrasive having a specific gravity of, say, 2.5 with another having a specific gravity of, say, 8.0... a good mental estimate can be made of how they compare with each other by visualizing where they fit among the four items in the list above.

Hardness

As is the case with density, it is an oversimplification to say that hard abrasives are used when maximum effect is desired on the substance being removed (rust, paint, etc,) and soft abrasives are used when minimum effect is wanted on the surface underneath-- particularly for vulnerable surfaces like aluminum, glass, etc. But again, that generally is true.

An obvious consideration is that a very soft abrasive would not etch a rough texture into a body panel surface but a very hard abrasive would. Even so, many performance characteristics related to hardness of abrasive are often governed by other factors such as shape.

Abrasive media hardness is usually specified by Mohs hardness numbers. The Mohs Hardness Scale is a 1-to-10 scale of hardness that was originally developed almost 200 years ago as a 'scratch-test' rating of the relative hardness of minerals. More details can be found in most any encyclopedia. The actual Mohs table lists ten minerals arranged here from hardest to softest.

While talc and diamond are substances known to everyone, some of these minerals are not as well known. Here is the same table but with the addition of some simple generic reference tests. There are any number of variations of these simple tests.

Recall that this is a 1-to-10 hardness scale. The minerals listed are used as standards to reference each hardness value. The numerals 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 in the list are the hardness values of each respective mineral standard. Other minerals and even many other substances can be fit within or on either end (i.e. lower than 1.0 or greater than 10.0) of this scale. For example, gypsum has a hardness value of 2. Gypsum is the substance used as a standard to establish the hardness value of 2. But... there are other minerals and substances that have a hardness value that is more than 1 but less than 3 making them a '2' also. Sometimes other substances-- meaning substances that are not one of the ten that are used as standards-- are rated to one or more decimal points. For instance, soda (sodium bicarbonate) is often specified as having a Mohs hardness rating of 2.5

Choosing an Abrasive and a Blast Process

Doing research about media characteristics in order to make an informed choice of an abrasive can be a real "where do I start?" frustration. One way to deal with this is to begin with a decision of how aggressive the abrasive itself needs to be before deciding how aggressive the blasting process overall needs to be. To that end it is very instructive to see a list containing both specific gravity numbers and Mohs hardness ratings of blast abrasive media. To help make the list more meaningful, keep in mind the following simplified lists for hardness and specific gravity.

Short list of specific gravity of common items

Abbreviated Mohs Hardness Scale

Here then is a list of common abrasive media. Sources differ slightly on exact numbers. This often has to do with the fact that any named abrasive may have several variations. One source may be stating specifications for one variant while another source is referring to a different variant. In some cases a named abrasive may actually be any number of only relatively similar substances that have been grouped together under one broad category. The term 'plastic media' is one example.

Harder (higher Mohs number), heavier (higher specific gravity) and therefore potentially more aggressive abrasives are in the upper portion of the list. Softer (lower Mohs number), lighter (lower specific gravity) and therefore potentially less aggressive abrasives are in the lower portion of the list.

Consider the two extremes. When the primary concern is to prevent damage to the item being blasted, abrasives that are soft and light are used. A likely example would be the use of soda to strip paint from light-gauge aluminum sheetmetal parts. When very hard-to-remove substances are being stripped from more or less indestructible items, abrasives that are very hard and extremely heavy are used. An example of this would be the use of steel grit for the blasting or wheel abrading of mill slag and weld slag from large, heavy, fabricated steel structures.

It is important to not confuse the aggressiveness of abrasive media itself with the aggressiveness of a blasting process using that media. For example, consider two abrasives having very different levels of aggressiveness as illustrated in the table above. One is heavier and harder. The other is lighter and softer. Now assume the lighter and softer abrasive is being used in #4 grit (very coarse) at 150 psi in a very large capacity outdoor pressure blaster having an extremely large blaster gun nozzle. Call this 'Blast Setup A'. Assume the heavier and harder abrasive is being used in #300 grit (very fine) at only 50 psi in a small blast cabinet with a small ID gun nozzle. Call this 'Blast Setup B'. Given its much more aggressive process (larger grit, higher air pressure, larger gun nozzle) 'Blast Setup A' is overall a much more aggressive method in spite of its less aggressive media (lighter and softer abrasive substance). In this hypothetical comparison, two widely different processes are used to make the distinction between blasting process and media more obvious. In real-world situations processes are often not so widely different and the distinction between process and media is less apparent. Keeping this distinction in mind may not make the process of selecting a media type a simple matter but it does help.

Sharpness, friability, and shape information could be added to the table but... for the most part, as used to rate abrasives, those three properties do not have numeric ratings. Sharpness and friability ratings are of the 'low-med-high' sort of rating or some variation of that. A description of shape for some abrasives is a simple one-word comment. For others though, shape is something not easily quantified into one single, simple description since some abrasives are available in more than one shape. Grit size is a specific and precise numerically assigned rating that could be included in the table but that would be of little value. That's because, excluding highly specialized media such as soda most abrasives are available in similar grit sizes. This was explained in the section above about grit size.

A Specific Example

An example of one way to make a choice about what abrasive/process to use is that of the blast cabinet that is the subject of this series of articles as it is set up when used to strip rust from body panels. Realize though that the choice described here is just one of many possibilities. Since there are so many variables involved with blast media, there seldom is one, single, 'correct' process for any given stripping task.

The process needs to be fast and efficient since it is for a commercial application. That is to say that if the customer is to be well-served, labor time needs to be minimized even if material cost is higher than some alternative. Trying to save the customer money by using a less expensive and less efficient blast media would end up costing the customer far more in longer labor times. While this might not mean choosing the highest-priced cost-is-no-object abrasive, it does mean that cost is secondary to speed. For a do-it-yourself hobby application, the reverse may often be true since hourly labor charges are not part of the equation. In this commercial application though, a relatively aggressive abrasive is needed in order to rapidly remove rust, weld slag and mill slag. Something from the top of the table in the previous section. Logical options are silicon carbide or aluminum oxide.

Silicon carbide and aluminum oxide are heavy abrasives but not so heavy as to create an even more critical air pressure versus work-hardening trade-off situation. Consider the fact that steel, with its extremely high density in comparison to most other media types, is the media of choice when work-hardening is actually the desired goal-- such as in shot-peening to create a harder, higher strength surface. For any given air pressure and grit size, steel grit would tend to work-harden sheetmetal more than silicon carbide or aluminum oxide.

Silicon carbide and aluminum oxide are harder than steel grit. That would mean potentially better ability than steel grit with respect to removing hard substances-- although abrasive friability is an issue with this as well.

Additional research will indicate that silicon carbide or aluminum oxide have other characteristics that are highly desirable for this application. They are very sharp. That should mean faster action on rust compared to a less sharp abrasive. They have reasonably low friability at low air pressure, so, they can be cycled through the blast cabinet repeatedly without being immediately pulverized into dust. They leave minimal abrasive contamination at the planned-on air pressure. These abrasives will leave a surface texture on sheetmetal panels but that is not a concern if that texture is very fine as a result of use of a very fine grit.  A very fine texture can easily be sanded off later if need be. However, no matter how fine the grit being used is, these abrasives are hard enough to ruin the finish of glass, chrome and other such surfaces. That is not a factor though since all such components are removed from the body panels prior to blasting those body panels.

A careful analysis of cost per pound, local availability versus freight charges, and life expectancy gives the nod to aluminum oxide.

Now that the abrasive type is chosen, the blast process is tuned to the application. As explained in detail earlier, a fine grit size is needed to 1) reduce individual abrasive particle weight in order minimize work-hardening, warpage, and coarseness of the surface texture... and... 2) reduce individual abrasive particle size to insure the abrasive can very efficiently scour rust out of the individual rust pits in the surface. A very low air pressure is used to further lower the tendency to warp or work-harden the sheetmetal. Blaster gun and air supply components having a higher than normal flow rate are used in order to regain some of the work speed lost to the lower air pressure setting.

For a typical rust stripping process used in this cabinet, the actual numbers are 220 grit aluminum oxide and 40 psi air pressure (during flow). By most standards for this type of job, this is very, very fine grit and very low air pressure. However, even at that low 40 psi downstream of the regulator, the cabinet's air supply system flows enough that it consumes 20+ CFM of shop air. This results in a reasonably good-sized abrasive stream. Actual stripping rate of heavy, deeply embedded rust is equivalent to, but actually much better in the long run, than a typical siphon sandblaster using 30 grit silica sand at 90-100 psi. 'Equivalent to' in that the initial rust removal occurs at about the same rate to perhaps slightly faster than the sandblaster. 'Better in the long run' in that the 220 grit aluminum oxide gets to the bottom of almost every pit on the first session, whereas 30 grit silica sand can require several sessions of blasting/disc sanding or wire-brushing as described previously. All this without the negative aspects of common sandblasting-- warpage, severe work-hardening, rough surface texture, and undue abrasive contamination of the surface.

End of Article