Bonding Material: While many sandpapers are bonded to the backing with hide glue, others use special waterproof glue or heat resistant adhesives.
Open vs. Closed Coat: Sandpaper may be either closed or open coat. Sandpaper with a closed coat is completely covered with grit while open coat sandpaper has small gaps between the particles of grit. While closed coat sandpaper gives a smoother finish, it tends to clog. The gaps between the particles in open coat allow dust to collect to prevent the paper from clogging.