Grit

• Sandpaper grit refers to the coarseness of the sandpaper; the lower the grit number, the rougher the sandpaper is. You can use coarse sandpaper to roughen a smooth surface or shape wood. You can use medium to extra fine sandpaper for smoothing surfaces.
  
  

Abrasives

• The type of surface you are sanding will determine the kind of abrasive you need. If you are sanding metal or hardwood, you will need a harder abrasive then if you are sanding soft wood. The most commonly used abrasives are flint, aluminum oxide, garnet, silicon carbide and ceramic.
  
  

Backing

• Sandpaper sheets and small sanding disks are made with a paper backing and sanding belts are made with cloth. Vulcanized fiber or Mylar is used when the sandpaper needs to have an inflexible backing.
  
  

Adhesives

• Most sandpaper uses hide glue to hold the abrasive onto the backing. Hide glue is not waterproof so you can't use it for wet sanding. You will need to get sandpaper that is made with a waterproof resin as the adhesive if you plan on sanding with a lubricant.
  
  

Open Coat or Closed Coat

• Open coat sandpaper has space between the individual grains of abrasive to help keep the sandpaper from clogging when you sand painted surfaces or soft woods. Closed coat sandpaper has abrasive covering the entire surface. Closed coat sandpaper works faster than open coat sandpaper, but it tends to clog.
  
  

Shapes

• Sandpaper comes in many shapes for use in different applications and tools. The most common shapes are sheets, disks, belts and rolls.