In a well-known auto painter time study, it was determined that those in the paint department spent one-third of their time at work sanding something. Metal shop techs are also frequently abrasively scrubbing away at the vehicle for one reason or another. While sandpaper may be a mundane item, it’s clearly a big part of what happens in collision repair. It is, in fact, a multi-point cutting tool designed to penetrate the substrate and pull the chipped material out of the scratch, leveling the old finish and providing increased surface area to improve the adhesion of the new coatings.
Automotive refinish sandpapers are manufactured all over the world. From Mexico to Portugal to Finland to Canada to the good ol’ USA, numerous manufacturers produce quality goods to expedite the refinishing process.
An educated consumer will understand the basics of something before they can appreciate the features (the unique characteristics of that brand), which lead to the advantages (better than the other choices because…) and finally the benefits (why your job will be easier, faster or better) of any particular brand. I’ll leave it to each vendor to convince you of their features and benefits, but will endeavor to inform you about the construction of automotive grade sandpapers.
From the Bottom Up
All abrasive papers and cloths begin with the backing material, on which is applied the first coat of adhesive, into which the abrasive particles are distributed. After their application, a second coat of adhesive is applied to hold the abrasive minerals upright on the backing and minimize clogging.
The residue from sanding is composed of both the abrasive minerals breaking down into dust and the sanded surface being removed. This mixture is correctly called “swarf.” Think of it this way: wet sanding provides the truest cut because the sheeting water washes the swarf out of the way of the next cut, preventing the clogging of the remaining abrasive minerals with hot, melted filler, primer, paint or clear. With the swarf out of the way, any sandpaper will last longer.
Most automotive refinishing sandpaper has a paper backing, although some types use foam or plastic film. Plastic film is an important development for waterborne/low-VOC base-coats, says Anne Knight of Carborundum Abrasives North America.
“Waterborne technology allows for thinner coats of paint, so it’s critical not to cut through them,” said Knight. “So if the substrate is totally flat (like film), then the ‘highs and lows’ of the grains are minimized and you get a more consistent cut, especially when using a tighter grain sizing.”
Heavy-duty abrasives can use a resin fiber backing, which is multiple layers of specially impregnated paper, and still others use cloth in various weights and stiffnesses to back the abrasive minerals.
Ordinary paper and cloth backings use a letter to delineate their thickness, strength and flexibility. By weighing a ream (480 sheets) of 24-inch by 36-inch paper, a weight in pounds is established and a letter is assigned. The lightest weight automotive paper backing is assigned the letter “A,” and heavier weights of thicker papers are rated “B,” “C,” “D,” “E” and “F.” In each case, the backing is made from heavier, thicker and more durable paper.
More durable backing papers are used with more durable and long-lasting abrasive particles. Putting a ceramic abrasive particle on an “A” weight paper would cause the paper to give out before the mineral was exhausted. Likewise, putting an inexpensive silicon carbide particle on “E” weight paper would be foolish, as the paper would outlast the mineral.
However, some manufacturers claim to have gotten around this paper weight-abrasive type issue.
“We offer a B-weight paper that’s reinforced with latex fibers to make it incredibly strong and tear-resistant, and that’s what we put our ceramic grain on…which is an exceedingly durable abrasive,” said Carborundum’s Knight. “Components can be added and formulations can be modified to enhance the strength and characteristic of a backing.”
Cloth backings are commonly used in grinding belts and are rated by their weight and flexibility. “J” weight cloth backing is very flexible and typically used in polishing operations. “X” weight cloth is heavier, stronger and less flexible, and “Y” weight cloth is extra heavy duty and used in the heavy grinding of bare metals.
Download a Sandpaper Spec Chart
Bottom of the Backing
Sandpaper is attached to the power sander or the sanding block in various ways. If the sandpaper is to be manually clipped onto the tool, then nothing but bare paper is on the bottom of the sandpaper. For many years, auto body techs applied glue from a squeeze tube or spray can to both the backing pad on the tool and the bottom side of the sanding disc to attach the sandpaper. This application of disc adhesive was time-consuming (the most expensive thing in any body shop is labor time!), and when a ribbon of glue from the tube wasn’t smeared flat, the paper wore out prematurely on the high spots of attachment glue.
When abrasive manufacturers began to coat the bottom of the sanding discs with pressure-sensitive adhesive (PSA), both the quick attachment times and the lack of high spots were a welcome change. Hook-and-loop attachment systems are not only fast and high-spot free, they run cooler because air can circulate under the sandpaper. Also, the paper can be easily removed and reattached, which is not always possible with many glued attachments. This is useful with 36 to 40 grit papers on airboard or hog sanders because it makes it easy to save and reuse sheets or discs that aren’t worn out.
The First Glue Coat
Correctly called the “make coat,” this adhesive is applied to the top of the paper backing by spraying, brushing or rolling it. It’s often made from animal hide glue or a man-made adhesive resin. When constructing wet-or-dry sandpaper, animal hide glue isn’t a good choice as the water will dissolve the bond and let the abrasive minerals come loose. How the adhesive is made and how it’s applied is one of the many differences in the shop’s cost of sandpapers. More expensive resin bond adhesives and precision application methods make for a sharper-cutting and longer-lasting sandpaper, but cost more than cheaper glues and less accurate application methods.
Abrasive Mineral Type
Some abrasive minerals are found in nature, others are constructed by mixing or synthesizing minerals or resins under high heat. Naturally occurring minerals like emery (fingernail files), garnet (balsawood airplanes) or flint (electronic contact points) are too soft for use in auto refinishing. We need an abrasive grain that’s hard, sharp and durable. Three basic manmade abrasives are used in our business:
Silicon Carbide. Silicon carbide is made from sand and carbon fused together in an electric furnace. It produces an abrasive mineral that fractures easily into sharp wedges. While those multiple sharp edges do a great job cutting, they’re not very durable as they just continue to fracture into smaller and smaller wedges. This is the mineral of choice for fine grit papers.
Aluminum Oxide. Aluminum oxide is a synthetic mineral made from bauxite, coke and iron fused together under varying temperatures; some is heat treated, some is not depending on the application. Its improved hardness fractures in larger chunks, cuts cooler and has a longer life than silicon carbide particles. However, because it cleaves in larger pieces, it doesn’t have as many extra sharp edges.
Ceramic. Ceramic abrasives combine the best of both worlds in that they’re very sharp, very hard, extra durable and run cooler. These particles may contain zirconium dioxide, which, when combined with alumina (used in the production of aluminum metal), creates a very hard particle that fractures in much smaller, sharper chunks. The actual formula for a particular brand’s ceramic abrasive mineral may be proprietary, and the manufacturer may have a specific name for their version. It usually takes more downward pressure on the sander to force the ceramic mineral particle to break down. In any case, these abrasive particles are created under the highest heat from the most sophisticated ingredients and cost the most to produce. Their micro structure won’t cleave or break away, which easily makes them very long-lasting.
As labor time is the most expensive thing in a body shop, any abrasive paper that lasts longer between disc changes will pay for itself quickly. If the cheap paper discs wear out and need to be changed every few minutes and the shop’s door rate is $42 per hour, then every minute the tech wastes changing discs more frequently costs 70 cents. Longer-lasting ceramic discs may quickly recover their additional costs.
Download a Sandpaper Spec Chart
Abrasive Mineral Size
Exactly how big the abrasive crystal is determines the “grit” number assigned to that size. There are three commonly used methods to grade particle size in automotive sandpaper. At 180 grit size, all three method’s particles are the same 0.00304 inches in diameter. Above and below that specific grit, there are slight differences in particle size, depending on which abrasive grading method is used.
ANSI. ANSI (American National Standards Institute) is the grading method most commonly used over the history of our business. With a 16-grit particle the largest and a 3,000-grit particle the smallest, these are the numbers we were raised with. To oversimplify, this method uses something like a kitchen sieve. Put a handful of abrasive particles in the sieve, shake it and everything the size of the sieve hole and smaller falls through, while larger particles remain in the sieve. The trouble with the ANSI system is that as much as 25 percent of the graded particles aren’t the same size. Common sense says that half will be smaller than the sieve hole and half that wiggle through will be larger. These coarser, “wild” grits can cause problems. If your 220 grit paper has some 280 mixed in, no problem. If your 220 paper has some 150 grit mixed in, that’s an issue. The larger particles may have slipped through the sieve sideways or snuck in.
FEPA. FEPA (Federation of European Producers of Abrasive) is a tighter grading system, and more of the particles are the same and uniform in size. You know when this method is used because the grit number is preceded by the letter “P” as P-220 or P-400. A 320 ANSI particle is about the same size as a FEPA P-400 particle. A P-1200 FEPA is about the same size as an ANSI 600.
JIS. JIS (Japanese Industrial Standard) is the tightest grading system commonly used in automotive sandpapers. Virtually all the particles are the exact same size when graded this way. This system uses a tighter grading system with less opportunity for “wild” grit to make it through to the sandpaper. There is no leading-letter indication that this is how the abrasive grains were sized; you would have to be told which system was employed (unlike the FEPA method, which always has the leading “P” before the grit size).
As an aside, grading abrasive particles in microns will produce an extremely uniform grit, as each one is exactly some exact micron in diameter. The problem is that we all know a smaller number is a coarser grit and a larger number is a finer grit (24 = coarse, 400 = finer). In microns, it goes the other way – a smaller number is a finer grit, and a larger number is a coarser grit (715 = coarse, 25 = fine). A confusing change; it’s hard to teach an old dog (or entire industry) a new trick.
Mineral on the Backing
Now with the paper backing weight chosen, the make coat of adhesive applied and the mineral and grit size established, it’s time to stick the abrasive into the glue. The cheapest method is to simply sprinkle the abrasive grains onto the backing, much like applying salt to your food or spreading grass seed by hand from a bucket. In this method, the grit sticks where it lands, and some particles will point up, down or sideways.
More expensive methods use an electrostatic charge to get the minerals to point up and coat the surface evenly. Whether the paper has a negative charge and the minerals a positive charge or the opposite, the minerals are attracted into the glue uniformly. Like every other step so far, more expensive choices make for longer-lasting sandpapers.
How much of the paper’s surface is covered with the abrasive mineral matters as well. When sanding something that will melt (body filler, primer surface, paint), you must have spaces in between the particles for the melted material to lodge momentarily until the next stroke of the sander can spin it out.
All automotive papers are “open coat” abrasives, meaning the minerals don’t cover the surface but rather have empty spaces between them. When grinding bare metal, “closed coat” abrasives are the choice because there are more of them and the bare metal won’t melt under the heat. To illustrate this point, take a painted part over to the bench grinder, which has a closed coat (solid abrasive) grinding wheel on it. Hold the painted part against the spinning wheel, and you’ll see that in moments, the melted paint covers the abrasive particles, clogs the minerals and prevents any more paint from being removed.
Download a Sandpaper Spec Chart
Second Glue Coat
Correctly called the “size coat,” this second application of adhesive has several purposes: to help hold the abrasive minerals upright, with the sharp edge out; as additional glue to physically hold the minerals onto the disc and not be broken off easily; and to help slide the swarf out of the way.
Untreated (no lubricant) sandpapers are often brown in color and could be called “production” paper. Lubricated papers have zinc stearate powder added to the size coat of adhesive and prevent the loading of the papers with swarf. Described as free-cut or no-fill, they last longer than untreated papers because they don’t clog as quickly. Zinc stearate is a slippery soap used in “fanning powder,” which magicians use to make a deck of cards fan out. This soft white powder is also used as a mold release agent to prevent the molded part from sticking to the mold in manufacturing.
One of the things that distinguish sandpaper brands is how well their anti-load lubricant works. Much like the production of ceramic abrasive particles is proprietary and unique to each manufacturer, their particular blend of anti-loading soap is, too. In an effort to distinguish improved versions from cheaper offerings within one brand and certainly between different brands, the manufacturer will add coloring to the final coat to distinguish their version. This is how refinish sandpapers came to be white, yellow, orange, pink, gold, green, blue, red, purple and any other color you might name.
One Among Many
In each case, the sandpaper manufacturer wants their blend of backing weight, adhesive type, mineral construction, particle size, mineral application method and lubricated adhesives to stand out from the other guy’s version. Like anything else in life, you get what you pay for. Lightweight papers, cheaper glues, softer minerals, causal grading systems and minimal lubricants cost less. Heavier backings, better resins, harder minerals, tighter grading and sophisticated lubricants cost more.
A productive body shop recognizes that techs’ labor time is far and away the biggest expenditure. That said, the smart choice is to buy the best, fastest cutting, longest lasting refinish sandpaper available. Your jobber will be happy to explain all the features, advantages and benefits of their brand…now that you know how it’s made!
Download a Sandpaper Spec Chart
Mark Clark is a well-known industry speaker and consultant. He’s celebrating his 23rd year as a contributing editor to BSB.