How to choose the right sandpaper/abrasive for your work?

Choosing the right sandpaper/abrasive (same knowledge can apply to sand disc as well) depends on the material you are using and the finish you wish to achieve. There are a few differences between silicon carbide and aluminum oxide (alumina), but both are popular options for metalwork and sanding wood surfaces due to their high performance levels, lower prices, and versatility.

If you weren't into earth science in high school, it might not be clear to you what these abrasive tools do. In this article, we compare the two options and discuss their best applications so you can make an informed decision.

How Does an Abrasive Grain Work?

A coarse grain of abrasive material wears down another material when it comes into contact with pressure. Sandpaper is one of the most common types of abrasive product, which manufacturers typically produce from silicon carbide (SiC) or aluminum oxide (Al2O3). Both of these types of abrasives are commonly used. More than 90% of the domestically produced abrasive grains come from these sources.

Other common abrasive materials include zirconia alumina, cubic boron nitride, synthetic diamonds, ceramic alumina, garnet, emery, quartz, silica sand, and crocus. Many of these abrasive grains mentioned are used to manufacture other types of abrasive products like grinding wheels, flap discs, flap wheels, and sanding belts.

Sandpaper grit must have a sharp shape and high hardness values to be abrasive. Grit numbers, which are used to describe the grain size, are often found next to them. Grit number describes the number of small openings per inch on the screen that the grain passes through. There are a variety of values from No. 4 to No. 2500, and particle sizes range from a few millimeters to less than two micrometers.

Bonded abrasives and sanding discs have a code on the label that tells you what kind of abrasive they contain. Aluminum oxide has the symbol A and silicon carbide has the symbol C. First on the left is the symbol, followed by the characteristics of the abrasive, including grit size and sometimes other characteristics. Below is an example.

By selecting an appropriate sandpaper/abrasive, the sanding and grinding process can be completed more quickly. If your material requires specific abrasives, techniques, or pressures, you can increase the surface finish. Abrasives with a high coefficient of friction reduce heat, which allows the abrasive to last longer and prevents the material from burning.

How Are Aluminum Oxide Abrasives Used?

In addition to working well on wood, metals, and painted surfaces, aluminum oxide sandpaper is also the most widely used grain. Additionally, it is more durable and cheaper to produce than other alternatives as well.

Different colors of aluminum oxide abrasives are available. Aluminum oxides are available in brown, white, and pink varieties, with brown being the most common.

Despite wearing faster than white or pink, they produce a smooth finish. Aluminum oxides such as these and silicon carbide are most comparable to each other. Aluminium oxide comes in a variety of textures, from coarse to fine. As it produces less heat, wood and lacquer are the best materials for using it. Between coats of finishing on wood, you can also use white grains. The pink aluminum oxide you can use on softer woods is available in a variety of textures.

Unlike pink and white aluminum oxide, brown aluminum oxide has a much longer shelf life, since the grain breaks down more slowly. The grits range from coarse to micron, and you can use it on metals, drywall, fiberglass, wood, and painted surfaces. With brown aluminum oxide you can grind your object more quickly and affordably than you can with pink or white aluminum oxide.

A wide range of aluminum oxides is available, including friable, semi-friable, regular, and heavy-duty. Rather than dulling down when used, friable abrasive grains resharpen, exposing sharp edges continually. As coarse grits continue to sharpen over time, they are able to remove metal stock, while finer grits are able to finish metal surfaces.

When you use aluminum oxide sanding belts, you'll find open and closed coats, as well as flexible closed coats. Because an open coat covers 60% - 65% of the grain, it is most effective on soft metals and woods.

Coatings that cover 90% - 95% of the belt surface are perfect for nonferrous metals and ferrous metals that are harder than steel. Last but not least, a flexible closed coat sanding belt can be used on woods and metals with recessed and curved surfaces.

Aluminum oxide sanding belts can only be used for dry operations. If you plan to use a metal grinding wheel, you can use aluminum oxide on materials with high tensile strengths, such as stainless steel, bronze, and aluminum alloys.

How Are Silicon Carbide Abrasives Used?

Sandpaper made from silicon carbide is the hardest and sharpest abrasive, but it lacks durability due to its brittleness. Additionally, narrower particles wear out quicker. The grains are razor-sharp, so they are able to sand metal, marble, glass, stone, cork, medium-density fiberboard, and plastic with little pressure. Despite this, they struggle to sand metal and wood.

On rough surfaces and for polishing, silicon carbide is an excellent choice. Compared to aluminum oxide, it has a higher level of friability, so it is suitable for wet sanding operations. Among the possibilities are polishing parts in automotive applications, removing rust, refinishing wood floors, deburring metal, smoothing glass edges, and sanding between final coats.

A combination of silicon carbide sandpaper and aluminum oxide is commonly used. Aluminum oxide abrasives are used for rough sanding, then silicon carbide is used for finishing. Using this technique, you can achieve a smooth surface finish without wearing your sandpaper down.

Silicon carbide is available in two forms, friable (green) and regular (black), with the friable variety being tougher and harder but more brittle.

The black silicon carbide can grind non-ferrous metals, ceramics, and hard nonmetals, while the green silicon carbide is best for polishing. However, silicon carbide should never be used for grinding steel.

Sanding belts made of silicon carbide are suitable for sanding harder materials such as wood finishes, stone, metal, and paint, and softer ones such as rubber, glass, and plastic. Microfracture (friability) makes the grains unsuitable for softwoods since they won't sharpen the edge with repeated use. You can use these belts for applying grain to a hard surface or finishing the surface with sealants and lacquers, since they have a closed coat that provides optimal grain coverage.