Sandpaper contains numerous sharp edges that cut away at wood or metal. The abrasive edges are glued onto backing material such as Kraft paper with a bonding agent. Different grades of sandpaper represent the difference in quality of the abrasives, the backing material and the bonding agents. Industrial-grade sandpaper uses higher quality materials than commercial grade but is only available in specific stores. In addition, sandpaper is measured by its grit size, or number of sharp particles per square inch of sandpaper. The larger the grit size, the more edges there are and the smoother the sandpaper. The density of the grit is important, too. Open-coated sandpaper has gaps between the grits, allowing sawdust to gather so it doesn’t interfere with the sanding, whereas closed-coated sandpaper doesn’t have those open spaces.
You need to choose the grit size of sandpaper depending on the particular job you are trying to accomplish. For heavy sanding and stripping, you need coarse sandpaper measuring 40- to 60-grit; for smoothing surfaces and removing small imperfections, choose 80- to 120-grit sandpaper. For finishing surfaces smoothly, use a super fine sandpaper with 360- to 600-grit. Many jobs require you to “go through the grits.” This means you start the project using lower-grade grit and use finer pieces of sandpaper as you progress. Each time you advance to a higher grit sandpaper, you remove the scratches from the previous layer.
There are four main types of sandpaper grits: aluminum oxide, garnet, silicon carbide and ceramic. Aluminum oxide lasts longer than the other kinds of grits since it contains a self-renewing property; because it's the most delicate, it crumbles easily, forming new soft edges. Garnet wears out the fastest but produces the smoothest surface. Silicon carbide is ideal for sanding harder materials such as metals and plastic. Finally, ceramic, the most expensive and roughest grit, is used for shaping wood.