Using silicon carbide abrasives for a finer finish

When you’re planning your wood project, it is important to remember that there are two separate sanding processes that will take place. In the first process, you will be sanding raw wood. Depending upon the condition of your stock when you receive it, you could begin as low as 24 grit with abrasive planing or dimensioning on a wide-belt sander or drum sander. Or you could start with some freshly planed material at 80 or 120 grit and climb up the grit sequence from there.

You will most likely use aluminum oxide (AO) abrasives as they are the most universal and economical of the more common abrasives available today. For most types of wood, and considering most finishes, sanding raw wood higher than 220 grit is not generally recommended, as the higher grits tend to burnish or polish the wood. When burnishing occurs, some of the wood’s pores are closed off and the burnished surface might not take stain evenly or allow sealer and topcoats to adhere to their fullest extent. And you still might need to raise the grain with a damp cloth and lightly sand it with the same grit you previously used.

Regardless of how fine you sand the raw wood, eventually, you will reach a point where you begin applying the finish. This is where your second sanding process begins: Sanding between coats of finish.

Side note: The first step at every stage of adding finish will always be the removal of any dust and debris from the sanded surface and any contours on the workpiece. There are several ways to do this effectively and the one you use comes down to a matter of preference. I like to grab the air hose and lightly rub my hand over the piece while blowing it off with the nozzle. The hand rubbing lets you feel any stubborn debris that doesn’t want to let go and allows you to gently dislodge it to be swept away in the current of air pressure from the nozzle. At the same time, it can help you identify surface imperfections that weren’t noticed during your visual inspection. A final rub-down with a tack cloth or a clean, soft paintbrush works for this process as well.

Now that the finishes are being applied to the workpiece, the sanding process provides a whole new set of demands. Whereas sanding raw wood requires more in the way of material removal, sanding finishes is more of a smoothing process that requires that you remove as little material as possible, while still sanding the surface smooth.

When sanding finishes, you are facing a different set of requirements. You need as fine and even a scratch pattern as you can get, and you need to ensure that the finish does not clog or load your sandpaper.

In order to understand the “how-to” of a fine scratch pattern, we need to examine the different abrasive grains and how they affect the finishing process.

Aluminum oxide is a tough blocky abrasive grain that works well for almost any application. While in use, it will gouge or plow out the material and it leaves a good scratch pattern. As the aluminum oxide abrasive grains wear, they begin to round off, not unlike the lead of a pencil. Some say that they sand finer and finer, but technically this is not the case. An 80-grit abrasive grain will always be an 80-grit abrasive grain. It is categorized as such because of its size. Even though it becomes shorter and shorter as it wears, it will still maintain its width. So, it does not become a “finer” abrasive grain. It simply will not dig as deep into the material as it did when new and untouched. It will be less aggressive.

The shape of the abrasive grains can also influence the uniformity of the scratch pattern. And even though aluminum oxide will do a good job of sanding coats of finish, the silicon carbide (SiC) abrasive is the preferred grain for this type of application.

Silicon carbide is a very hard abrasive grain. So hard, in fact, that it is considered second in hardness, with only diamond abrasives being harder. Therefore, it is the preferred abrasive for the glass industry.

Along with being a very hard abrasive, silicon carbide has a couple more characteristics that lend it to sanding between coats of finish. Minerals that are very hard are usually very sharp, as well. Such is the case with silicon carbide. And because this mineral is so sharp, it performs more of a cutting action than aluminum oxide with its plowing or gouging scratch. This means that it will leave a much cleaner cut than the AO. And a series of cleaner cuts equate to a more refined and more even scratch pattern, and a better finish.

The SiC mineral is also friable. This means that as the abrasive grains are in use they continually fracture and re-sharpen themselves. This process requires light to moderate pressure and adds a benefit that is very valuable to the finish-sanding process.

Because of the continual sharpening of the abrasives, this mineral provides the same aggressive scratch pattern throughout the entire life of the abrasive material. So, you get a consistent scratch the whole time, which is perfect when smoothing coats of finish.

And finally, the backing comes into play, as well. Cloth backings are not recommended for finishing grits 150 and up. The cloth backings are made up of interwoven fibers or threads that make for a somewhat uneven surface for the application of abrasive grains. That is why cloth backings are recommended for material removal and intermediate grits from 24-120.

Paper backings are much smoother than cloth and make for a more even surface to which the abrasive grains adhere. Some paper backings are latex impregnated, which makes them waterproof.

Polyester film backings are even smoother than paper. They provide an ultra-flat surface along with toughness, and durability, and are waterproof for use in wet sanding applications, as well.

Silicon carbide abrasive grains on either paper, latex-modified paper, or film backings are all combinations that cannot be beaten for smoothing those coats of finish, helping to make your final project look gorgeous.  

This article was originally published in the March 2023 issue.