I use sandpaper throughout the stages of building a project — fine-tuning a joint, easing edges, smoothing panels, and sanding between coats of finish. But there’s more to sandpaper than getting smooth surfaces. Understanding how different types of sandpaper work helps you choose the right material for the job and get better, faster results.
When it comes to sandpaper for woodworking, there are four commonly used abrasives — garnet, aluminum oxide, silicon carbide, and alumina zirconia. And each type has its own unique characteristics and working properties, as you can see from the chart.
FRIABILITY. Some abrasives break easily when heat or pressure are applied. This is know as friability . Depending on the amount of breakage, the abrasive is said to have a high or low friability rate. Highly friable abrasives break down easily, continually creating new, sharp surfaces. Abrasives with low friability tend to dull or round over faster.
HARDNESS. Another important factor in sandpaper is the relative hardness of the abrasive material. The different types of sandpaper have varying degrees of hardness. (Hardness is measured by testing with a diamond.) But hardness is no indication of friability, as you’ll see.
GARNET. Garnet is a naturally occurring mineral used as gemstones and as an abrasive. It has a low friability rate, so the tiny grains on this sandpaper don’t break. But that doesn’t mean the paper lasts longer during use. In fact, it’s the opposite.
Garnet tends to round over quickly, which means the grains won’t cut as effectively after they’ve dulled. But rounded grains aren’t entirely bad when you’re sanding bare wood.
Garnet’s limited friability actually makes the sandpaper better at sanding end grain. Because the grit dulls easily, it does a nice job of burnishing the wood rather than cutting through it, like other sandpaper abrasives. Burnishing seals the end grain and results in a cleaner surface. Plus, the burnishing effect also produces a softer scratch pattern, which results in a smoother feel to the sanded bare wood.
The wear factor of this sandpaper makes it a poor choice for power sanding. You won’t often find it made into disks or belts for power tools. For this reason it’s best used for final hand sanding to create a smooth, satiny surface.
Although garnet isn’t available for power tools, you will find it in sheets. And of the four types of sandpaper discussed here, it’s the least expensive to buy.
ALUMINUM OXIDE. Aluminum oxide is the most common mineral used to make sandpaper for woodworking. This man-made mineral is as hard as garnet but it has a higher rate of friability. The particles break easily during use, creating renewed cutting edges. As a result, it lasts longer and sands just about any material — bare wood, painted surfaces, metal, or plastic. Because of its versatility, aluminum oxide is the workhorse of the woodworking shop.
But there are drawbacks to this type of paper too. Because the grains fracture so easily, some grains become smaller and others remain large, which results in an uneven scratch pattern on the wood surface.
Another drawback is the sharpness of the grit. Aluminum oxide is not as sharp as garnet. The particles have a rounded, chunky shape. But even at that, it still produces a surface that will take a finish well.
SILICON CARBIDE. Silicon carbide is the sharpest mineral of the four mentioned here. The grains are glass-like shards with super-sharp edges that make it great for cutting through dried finishes.
The particles break down easily and create new edges when used on hard surfaces. These edges are good at removing rust from metal, old finish from wood, or sanding between finish coats, but not so good on bare wood.
The tiny, sharp edges on the mineral also produce a uniform scratch pattern. Another reason it’s good between coats of finish.
Silicon carbide sometimes has a cloth or waterresistant paper back, so it can be used with a lubricant like water or oil. The backing and ultra-fine grits (up to 3000 grit) make it a good choice for sharpening tools or wetsanding a finish.
ALUMINA ZIRCONIA. Alumina zirconia (also known as ceramic) is another man-made mineral that really lasts. It withstands high temperatures and heavy use, so it’s often used on sanding belts. I usually use it for leveling and shaping because its tough nature lets me remove a lot of wood at once.
A sanding belt coated with alumina zirconia will last longer than most other abrasives. So even though it costs a little more, you won’t have to replace it as often as other types of sandpaper.
If you keep a variety of grits on hand in these four types of sandpaper, you’ll have what you need for almost every task in your shop. And the results will show in your finished projects.
Open vs. Closed-Coat Paper
There are two ways to apply grit to paper — open-coat and closed-coat. These terms refer to the spacing of the individual grains of abrasive, which has a direct bearing on the useful life of the sandpaper.
Open-coat paper has wide spaces between the grains. These spaces acts like the gullets on a saw blade and allow the dust to fall away instead of clogging the paper. It helps the sandpaper last longer.
The grit on closed-coat paper is tightly packed. It’s used where loading isn’t an issue, such as when sanding metal. One advantage to closed-coat paper is that it produces a more even scratch pattern.
