Grit

• One of most important qualities of sandpaper is the level of "grit," or the amount of abrasive material per square inch of paper. The higher the number, the more "grit," the paper is said to have, ranging from coarse to super fine.
  
  

Open and Closed Coats

• In addition to a difference in grits, sandpaper can be classified by whether it is "open coat" or "closed coat." Open coat refers to the spaces between grits, meant to help sawdust move easily between them. Since closed-coat sandpaper would become clogged with sawdust if used on wood, it is often employed instead in metalworking.
  
  

Other Features

• Other features that distinguish different types of sandpaper are backing material and bonding agents. Backing material can either be made of low-grade fabric or higher-grade cotton and polyester. Bonding agents attach the backing material to the sandpaper---ones of poorer quality tend to use glue that easily disintegrates.