Knowing how it works is the first step in choosing the right abrasive

by Strother Purdy

photoYears ago at a garage sale, I bought a pile of no-name sandpaper for just pennies a sheet. I got it home. I sanded with it, but nothing came off the wood. Sanding harder, the grit came off the paper. It didn't even burn very well in my wood stove.

Sanding is necessary drudge work, improved only by spending less time doing it. As I learned, you can't go right buying cheap stuff, but it's still easy to go wrong with the best sandpaper that's available. Not long ago, for example, I tried to take the finish off some maple flooring. Even though I was armed with premium-grade, 50-grit aluminum-oxide belts, the work took far too long. It wasn't that the belts were bad. I was simply using the wrong abrasive for the job. A 36-grit ceramic belt would have cut my sanding time substantially.

The key to choosing the right sandpaper is knowing how the many different kinds of sandpaper work. Each component, not just the grit, contributes to the sandpaper's performance, determining how quickly it works, how long it lasts and how smooth the results will be. If you know how the components work together, you'll be able to choose your sandpaper wisely, and use it efficiently. Then you won't waste time sanding or end up burning the stuff in your wood stove.

Sandpaper is a cutting tool

What sandpaper does to wood is really no different from what a saw, a plane or a chisel does. They all have sharp points or edges that cut wood fibers. Sandpaper's cutting is simply on a much smaller scale. The only substantial difference between sandpaper and other cutting tools is that sandpaper can't be sharpened.


Sandpaper in cross section

Sandpaper is made of abrasive minerals, adhesive and a cloth, paper or polyester backing. The abrasive minerals are bonded to the backing by two coats of adhesive; first the make coat bonds them to the backing; then the size coat locks them in position.

Look at sandpaper up close, and you'll see that the sharp tips of the abrasive grains look like small, irregularly shaped sawteeth (see the drawing above). The grains are supported by a cloth or paper backing and two adhesive bonds, much the way that sawteeth are supported by the sawblade. As sandpaper is pushed across wood, the abrasive grains dig into the surface and cut out minute shavings, which are called swarf in industry jargon. To the naked eye, these shavings look like fine dust. Magnified, they look like the shavings produced by saws or other cutting tools.

photo 10cleardot.gif (49 bytes) Magnified above, swarf from sanding with the grain looks like shavings from a ripsaw

Even the spaces between the abrasive grains serve an important role. They work the way gullets on sawblades do, giving the shavings a place to go. This is why sandpaper designed for wood has what's called an open coat, where only 40% to 70% of the backing is covered with abrasive. The spaces in an open coat are hard to see in fine grits but are very obvious in coarse grades.

Closed-coat sandpaper, where the backing is entirely covered with abrasive, is not appropriate for sanding wood because the swarf has no place to go and quickly clogs the paper. Closed-coat sandpaper is more appropriate on other materials such as steel and glass because the particles of swarf are much smaller.

Some sandpaper is advertised as non-loading, or stearated. These papers are covered with a substance called zinc stearate--soap, really--which helps keep the sandpaper from clogging with swarf. Stearated papers are only useful for sanding finishes and resinous woods. Wood resin and most finishes will become molten from the heat generated by sanding, even hand-sanding. In this state, these substances are very sticky, and given the chance, they will firmly glue themselves to the sandpaper. Stearates work by attaching to the molten swarf, making it slippery, not sticky, and preventing it from bonding to the sandpaper.

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