Sandpaper for Autobody Repair

The December 2011 issue of BodyShop Business Magazine features a Tech Editorial on the sandpaper used in bodyshops for crash repair. Since Abrasive Resource is one of the suppliers referenced, we are happy to share the article written by Mark Clark here:

"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.

Universal Manufacture

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.

The Backing

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.

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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.

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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.

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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!

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Mark Clark is a well-known industry speaker and consultant. He’s celebrating his 23rd year as a contributing editor to BodyShopBusiness. To read this article in the context of the magazine and to access an additional Sandpaper Spec Chart, please visit the BodyShop Business Website!

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Buying Your First Sander

My son is at the age now where he is setting up his first workshop and is ready to move on from always borrowing Dad's tools to owning his own. Recently, ShopNotes ran an article in their "Setting Up Shop" series devoted to buying your first sander--I wish I had written it, because I agree with everything they said!

When it comes to buying your first sander, my suggestion is to get a 5" random orbit sander. This tool provides a good balance of features to tackle all kinds of tasks. And once you get an idea of its capabilities and limitations, you can use that information to guide your decision for buying other sanders.

Because tool models are always changing, I usually hesitate to recommend a certain brand or model. Instead, I like to look for specific features. So here are the things I feel are important in a sander:

The Sanding Pad- The 5" diameter pad is small enough to use one-handed but large enough to keep surfaces level. Another thing I look for is a hook and loop pad. Switching (and reusing) sanding disks is just so much more convenient than pressure sensitive adhesive (PSA) disks. Note: you can buy hook and loop replacement pads for most PSA sanders. Abrasive Resource carries these on our website:
5" Conversion Pads .

Dust Collection- Also high on my list of priorities is dust collection. Some tasks, like flattening a panel, can generate a lot of dust. Keeping it out of the air (and my lungs) is important. Although all sanders come with some kind of filter, I look for sanders that offer easy hook up to a dust collector or shop vacuum.

Comfort- Finally, I look for a sander that feels comfortable. Admittedly, this is the most subjective "feature". You'll need to get your hands on several models to find out which one is a good fit. Some sanders offer two hand positions: a top mounted handle or a center body grip.

Now all that's left is the sanding discs! Abrasive Resource converts discs in any size, grit, backing and vacuum hole pattern in both the self adhesive PSA style, as well as the hook and loop backed discs. Visit our random orbit sanding disc page and give us a call at 800-814-7358 if you have any questions! Sanding Discs from Abrasive Resource

How to Achieve a High Gloss Varnish Finish on your Wood Projects

After sanding your wood piece with a final sand of P220 grit paper, remove the dust using either a vacuum or a tack cloth. Apply a coat of orange shellac and let it dry for 30-45 minutes. Now apply your first coat of varnish and let it dry for 12-24 hours.

If you apply the second coat the next day, there shouldn’t be any need to sand the first coat because the second will bond chemically to the first. If, however, more than a day has gone by since the first coat, the second coat will no longer chemically bond to the first, so you will need to scuff sand the first coat with P220-P320 grit sandpaper to give that second coat a physical bond.

Now it’s time to level the first two coats. After the second coat has dried for at least 24 hours, sand it with P220 grit sandpaper. Sand with the grain, using a rubber or cork sanding block on flat surfaces to keep the pressure uniform. Use your fingers only when you can find nothing else that will work as a sanding block—and switch to P320 grit so that you don’t cut through on edges or details!

As you level the surface, you should reveal a pattern of dull and shiny areas…the dull areas are the high spots being planed down by the abrasive and the shiny areas are the low spots that haven’t been touched. Don’t try to remove all the shiny areas completely, because you may accidently cut through on the edges.

After removing the sanding dust, apply a third coat of varnish. Let it dry for 12 to 24 hours. Now you’ll need to make a judgment call—do you have an unfilled pore pattern showing or do you feel that your first three coats were fairly thin? If so, now is the time to apply a fourth coat directly over the third. Let the fourth coat dry for 48 hours.

Sand this third or fourth coat with P400 grit SC Waterproof sandpaper, again using a sanding block. Lubricate the paper with water, adding a drop or two of dishwashing detergent to prevent clogging. Frequently wipe away the slurry to check your progress. If the varnish is making little balls on the sandpaper, that is an indication that your varnish needs to dry longer—even several days! Look for the same dull/gloss pattern as before, but aim for having fewer shiny areas and a flatter surface. Wipe down the piece with mineral spirits to remove any dried slurry.

For the final coat, thin your varnish with about 15% mineral spirits so that it flows on with virtually no brush marks, filling any hollows. Now let this final coat dry for at least 48 hours—but even longer is better. The harder the finish, the smoother the final result will be!

Now it’s time for your finish sanding. Begin with a light wet sanding in P1000 grit SC Waterproof sandpaper to remove the dust nibs and any small brush marks. If the sandpaper starts clogging, stop sanding and wait another day or two for additional drying of the varnish. Once you are in this final sanding stage, regularly check your progress until 80-90% of the surface is dull. Go light on the edges and small details.

Next, wipe down the piece with a damp sponge and clean water. Repeat the sanding process with sandpaper in P2000 & P3000, being careful to always clean the surface entirely before moving onto the next grit. You should only have a light sprinkling of low, shiny freckles left behind on the flat surfaces. You can continue this process all the way up to a P 5000 grit sandpaper finish if you would like! Finish by rubbing out to the desired sheen with a wax.

You can find all of the sandpaper mentioned in this blog post on Abrasive Resource's Online Store.

Abrasive Resource introduces the new “Big Foot” Sanding System

Abrasive Resource, a leading US converter and distributor of abrasive sanding supplies, is pleased to announce its’ expansion into manufacturing drive pads and custom back-up pads for use on the GEM® Orbital Sander/Polisher. These slip on pads, along with 11 x 5 or 12 x 6 hook and loop sanding discs, make up the Big Foot Sanding System.

Abrasive Resource has served the wood and solid surface markets with sanding discs for over 25 years and the addition of custom back-up pads designed for use on the GEM Industries® Orbital Sander was a natural progression in their offering of abrasive products and supplies. Now, in addition to distributing the popular GEM sander, Abrasive Resource will also provide fabricators and shops with options of an 11” diameter pad with a 5” center hole or a 12” diameter pad with a 6” center hole.

The Big Foot Sanding System allows the sanding pressure to be concentrated on the outside diameter of the back-up pad. This provides a uniform scratch pattern and relieves any loading that may be created in the center of a traditional solid sanding disc. The slip on back up pads are tapered on the edge to enable finishing all of the way up to a back splash. In addition, the 5” or 6” center cut out discs are available with any vacuum hole pattern and are included at no charge to use with a shop’s smaller disc sanders.

To learn more about the “Big Foot” Sanding System, visit www.abrasiveresource.com.

Introductory kits are available in the 11” x 5” size at http://www.abrasiveresource.com/detail_sale115BPDDVKIT__DonutDiscs__Blue_Hook_and_Loop_Sanding_Discs.html .

The 12” x 6” size can be found at http://www.abrasiveresource.com/detail_sale126BPDDVKIT__126bluhookdisc__Blue_Hook_and_Loop_Sanding_Discs.html .

Media Contact:
Debbie Swanson
Abrasive Resource
900 Lund Boulevard #100
Minneapolis, MN 55303
mail@abrasiveresource.com

Choosing the Correct Backing for Your Sandpaper & Abrasive Products

Whether paper, cloth or film, the backing used for coated abrasive products must be smooth enough for a uniform adhesive coating, strong enough to withstand grinding pressures and flexible enough to conform to contours if that is necessary to your application.

Paper: The paper backings used for coated abrasives are highly specialized technical papers made to very exact specifications for abrasive use, thereby assuring certain essential physical properties such as finish, strength, adhesion, flexibility and weight.

Paper weights are determined by the number of pounds in a papermaker's ream of 480 sheets that are 24" x 36" in size. The standard paper weights used in coated abrasives are shown below and are indicated by a letter code which appears immediately after the grit size on the finished product backing. Briefly stated, the lighter the backing, the greater the degree of flexibility and the heavier the backing, the greater resistance to tearing.

• A-weight (40 lbs) -Light and flexible. A weight is primarily used for sheet products used on hand sanding operations in grits 80 and finer.
• C-weight (70 lbs) -Stronger and less flexible than A weight. This backing is chosen for hand sanding and for use on small, portable power sanders. Used for intermediate sanding in grits 36 through 80.
• D-weight (90 lbs) -Stronger and less flexible than C weight. This backing is also used for hand sanding and small, portable power sanders on coarse and intermediate grits 24-80.
• E-weight (130 lbs) -Stronger and less flexible than D-weight, this backing is primarily used on roll, belt and disc applications where high resistance to tearing is needed.
• F-weight (165 lbs) -The strongest, least flexible paper backing utilized. Used primarily for heavy-duty sanding discs, sheet goods and sanding belts used in the floor sanding market and paper wide belts used in woodworking applications.
  

Cloth: Cloth backings are more durable than paper, offer greater resistance to tearing, and tolerate continual bending and flexing during use. The cloth used for abrasive backings are manufactured in the traditional way most cloth is processed--by interlacing threads running at 90' angles with each other. The friction at the numerous points of intersection hold the fabric together. Cloth used for abrasive backings often requires desizing, shrinking, drying, stretching, filling and calendering in order to produce the desired strength, flexibility and coating surface.

The standard cloth weights used in coated abrasives are also indicated by a letter code which appears immediately after the grit size on the finished product backing.

• J-weight (Jeans) -The lightest and most flexible cloth backing, this backing is used where finish and uniformity of surface are more important than stock removal. Most often seen on abrasive shop/utility rolls, cloth sheets and sanding belts. Ideal for finishing, blending and where considerable flexibility and conformity are required, such as contour work on curved surfaces.
• X-weight (Drills) -Stronger and stiffer than J-weight, this backing is used on a multitude of abrasive products from coarse grit stock removal through fine grit finishing and polishing. Consistent productivity, good finishes and long product life are characteristics of abrasive products made on an x-weight backing such as sanding belts and cloth sanding discs.
• Y-weight (Heavy Duty) -The strongest cloth backing utilized, this backing is used on coarse grit products designed for extreme pressure and heavy stock removal operations. Used most often in heavy duty, long life premium abrasive belt materials.

Polyester Film: Polyester based film backings offer special properties that lend themselves to specialized sanding applications. Film backed abrasives offer thicknesses having ultra tight tolerances...which is important for the electronics, fiber optics and surgical instrument/medical applications. The benefits of a film backed abrasive product is now also being realized in the automotive and solid surface markets.

Sanding Wood

Wood surfaces are not only sanded to make the surface smooth! Although that is the most important reason, there is another factor to take into consideration: wettability.

This concept is easily demonstrated with The Water Drop Test. The photo to the right shows the differences in wettability on a piece of yellow birch. The two arrows are pointing to the inactive and activated surfaces.

The test has three drops of water placed on the surface at the same time. The drops were then photographed after 30 seconds. The water on the left was dropped on an unsanded wood surface and as you can see, it has kept its large contact angle with the wood surface. The surface under the middle drop was sanded with two passes of 220 grit sandpaper and you can easily see that the wettability is much improved. The last drop has almost completely soaked into the surface. Why? This section of the wood was treated to five passes of the 220 grit sandpaper.

So, why is this important? Because it reminds us that when sanding the surface of wood you are not only making the surface smoother. You are also reactivating the wood. Both time and heat cause extractives like tannins, oils & resins from within the wood to rise to the surface. Think about how this may affect your finish on the wood. Bottom line: All wood should be sanded no greater than 24 hours before it is finished and sealed.

Properly sanded surfaces result in beautiful finished products. You will save time and money by reducing the amount of finish needed--one would have to use only a fraction of the coating or stain on an activated surface! And even better is that by allowing the stain to wick out evenly you achieve a much more uniform color and finish.

Source: WoodworkingNetwork.com
For more information on sanding wood or any other surfaces, please give Abrasive Resource a call at 800-814-7358 or check us out online at AbrasiveResource.com