Despite its name, sandpaper is not made of sand. Sandpaper is actually a sheet of abrasive material glued to a piece of paper. The grit of the sandpaper, the weight of its paper backing, and the type of abrasive material affects how sandpaper works. Sandpaper works by using abrasive particles to remove the surface of materials, usually wood. Applying pressure to a piece of sandpaper either directly with your hand or with a sanding block and moving it over a surface causes the particles to act like the teeth of a saw, cutting off bits of the material. The 'sharpness' of the sandpaper is determined by its grit.
Grit and Coating
Grit is the number of abrasive particles on an inch of sandpaper. A piece of sandpaper with a high grit gives a smoother finish. A piece of sandpaper with a low grit gives a rougher finish. For example, a 40-grit sandpaper is very coarse while a 600-grit sandpaper is extremely fine.
The abrasive particles have two types of coatings: open and closed. Open coat means that there are more spaces between the particles. This is best used for soft metals, woods and painted surfaces. Open coating allows for more pieces of the abraded material to slough off. Closed coat has fewer spaces between the particles and are best used on very hard metals and wood. With fewer spaces closed coat sandpaper clogs up faster.
The actual grit of the sandpaper is glued to pieces of paper which have four weights. Weights are determined by the amount of pressure the paper can withstand before tearing. The lightest weight is A, followed by C, D and E which is the heaviest weight. So if you were scraping paint from a metal tub, you would want to use E paper so that the paper won't tear against the harsh surface from the pressure of your movements.
Types of Sandpaper
Sandpaper's abrasive quality is also affected by its type. There are four main types of sandpaper, in order from hardest to softest: ceramic, silicon carbide, garnet and aluminum oxide. Ceramic is the hardest material and is used to quickly remove surfaces. Silicon carbide is the next hardest, and is used to grind away metal, fiberglass, plastic and paint. Garnet is used to produce a smooth finish and burnish wood. Aluminum oxide is the longest lasting of the four types and also the most common.