One well-known time study of auto-mobile painters showed that the typical painter in a collision repair facility spent one-third of his or her time sanding something. He or she spent another third painting something and split the last third evenly between prep and detail.

In a 40-hour workweek, the painter spent 13.3 hours sanding. At a $40 per hour door rate, that translates into $532 of labor time per week, or at $50 per hour, $665 of labor. Everyone makes more money if the technician can beat the flat-rate times. I’m sympathetic to the argument that good collision repair is part art and part science, but it seems to me that any prudent painter would do his or her best to minimize the time spent pushing a rubber block across the car.

“But I can’t tell if the panel is ready for paint until I touch it,” says the artist. “Besides, power sanders are expensive, and foam pads and rubber blocks are cheap.”

If our target painter could cut sanding time by 10 percent by employing a power sander, at a $40 per hour door rate, he or she (and the shop) would be ahead by $53 each week or $227.90 each month. The tech could purchase a nice, brand-new sander every month with the time saved. At $50 per hour and a time savings of 20 percent (closer to reality, in my experience), they’re $133 ahead each week and $572 ahead each month. The magic words in any body shop are “labor time,” and mechanizing every task will help get the work done faster and make both the tech and the shop more money.

Skipping right to the point of this article, stop hand sanding and use an air-powered sander at every opportunity! Yes, I understand that the styling lines and some portion of a scuff and buff on the clearcoat will require some handwork. The object is to minimize the necessity to sand anything with a rubber block. Power sanders are much faster, and fast is the name of the game in collision repair.

I have arbitrarily divided auto body power sanders into three categories. Common sense says there will be significant overlap among them, but for the purposes of this article we’ll speak about power sanders as those that: 1) remove material, 2) shape the surface and 3) final finish the work. Along the way, we’ll talk about why quality tools are less fatiguing than throwaways, and why wet sanding might still be a useful methodology.

Removing Material

This is the portion of the work that’s seldom done by hand by anyone. All you vo-tech grads probably still have the long board (a.k.a. the idiot stick) that you started school with and maybe even drag  across a troublesome panel once or twice a year, but in large part everyone recognizes that removing old finishes or shaping body filler is a job for power tools. This rough work can be accomplished quickly by using a:

• Grinder. While the 24 or 36 grit 7-inch grinding disc will rip material off the surface fast, it also generates lots of heat. Any tool spinning at 3,000 to 5,000 rpm gets the sandpaper and the surface really hot. When you get the old paint or new body filler hot, it melts and clogs the expensive grinding disc. Let’s not forget also that it takes a manly man to shape filler with any kind of finesse when the tool is heavy and whirling at 5,000 rpm.

The other part of the equation is that once the material is ripped off with the grinder, the tech now has to fill and smooth those 24 grit scratches, adding more labor time to the repair. A better choice to strip paint and shave filler with 36, 40, 50, 60 or 80 grit paper might be a hog sander.

• 8-inch hog sander. This tool operates at a much lower rpm (900 to 1,000) and therefore runs much cooler. Because it spins slower, you can sand off the material with a paper disc rather than cloth. Materials are much cheaper. Whether this style of tool runs in a rotary (straight circle) or a rosette (offset circle) pattern, it removes the old finish or the body filler quickly and with lots of control.

To get the maximum benefit, make sure to hold the sander flat against the surface rather than up on the edge like a grinding disc. There are still some techs who think that to keep a long panel straight, they need a tool with a longer pad than the 8-inch round ones found on this style of sander. They prefer to use a file board.

• File board. With a 2-3/4-inch by 16-1/2-inch pad, this powered “idiot stick” is the staple of the industry. If you’re old enough, you remember when this tool was equipped with a 14-inch metal file rather than a sanding pad, and metal men used them to power-file down high spots in the steel and shave body solder (lead). Hence the name “file board.” Today, virtually all file boards are equipped with sanding pads. These tools can run in a straight line (back and forth) and be powered by a single piston with two ringed ends or by two pistons, one to push the pad forward and the other to push it back. Or they can run with an orbital wiggle (jitterbug) pattern.

Both styles have their advocates and strengths. In either case, it’s a great way to keep a long panel straight and chop off unwanted material quickly. Keeping piston-driven air files well lubricated will minimize the stalling of the pad. When the air pressure is equal on both ends of the stroke, the tool won’t start up when the trigger is depressed the next time. Many techs slam the end of the sanding pad to get the tool started back up, but keeping the cylinders well oiled is the key. And just a light push on one end or the other will start the tool back up smoothly.

Shaping the Surface

Shaping sanders are tools designed to prepare surfaces for paint. Power sanders do this rapidly, increasing the adhesion of the new coatings and featheredging the chips in the old finish. A smaller version of a file board sander with a straight line action could also work well to block the surface flat.

Prior to 1957, this type of sanding was often done by an orbital sander with a 3-inch by 8-inch or 3-2/3-inch by 9-inch pad called a jitterbug due to its wobbling action. Orbital sanders with various size pads are still useful today. Several sander manufacturers offer square or rectangular pads on a drive motor with a 3/16-inch orbit. In fact, most tools in this category (based on my arbitrary definitions) have a 3/16-inch or larger (3/8-inch) stroke – fast and aggressive enough to quickly shape and prepare the surface.

Just a reminder about how important thorough sanding is to ensure adhesion of the new coatings. It’s possible to double the contact of the surface area by sanding it well. How can that be? Imagine a distance on a flat surface from point A to point B. Now sand that same area and the distance from A to B (the contact area for the new coating), including the surface on both sides of the scratch (trench). Down one side of the scratch, up the other, down the next, up the other side – you get the idea. It will provide twice the surface to stick the new finish to.

A rotary or orbital tool runs the pad in a circle. A random orbit sander runs the pad in two circles, one inside the other. In 1957, the National Detroit Company patented the first random orbit sander widely used in refinish. In an effort to describe the circle-within-a-circle pattern, they called it “Dual Action.” It has become so pervasive in the market that its brand name has become a generic term. So when you say “D/A,” you likely mean a random orbit sander, now made by dozens of quality companies – kind of like when you say Kleenex and mean facial tissue, originally made by the Kimberly-Clark Company, or when you say Bondo® (a trademark of 3M Company) and mean polyester body filler, originally made and successfully marketed by the Bondo Corporation (now 3M Company). Or, if you’re in the South and say Coke, you really mean a carbonated soda made by any one of dozens of brands!

In this middle category of power sanders, most random orbit tools use either a 3/8-inch (very aggressive) or 3/16-inch (less aggressive but still fast) offset. The size of the offset determines how far the pad travels in each rotation. Bigger offsets cover more area but cause the tool to vibrate more.
In addition, some of these tools have a built-in wobble in the pad that interrupts contact with the work surface, and with the pad always changing planes, the sandpaper doesn’t load up as quickly and the pad runs cooler. What sort of wobble? Take a dinner plate, set it on its edge and spin it like a top. As the plate slows down, it will wobble through the last few rotations just before it stops and lays flat again. The pad works the same way, lifting its edges at the outside, allowing the sanding residue to be cast out from under the pad.

Sandpaper grits that would be used in this shaping tool segment run from 80 to about 400. By the way, there’s a two-grit jump from hand sanding to machine sanding. For example, if the tech hand sands with 400 grit, he or she can get the same scratch by using 320 grit on a random orbit power sander. The power tool moves much faster. As another example, if the tech runs 180 grit on the D/A, he or she will need to move up two grits to 240 grit to get the same scratch by hand. In many shops, it’s common to see the painter run a random orbit sander with 320 grit and then hand sand the same area with 400. Hello, it’s the same scratch! So stop hand sanding, or at least move to 500 or 600 grit to see a finer result.