Grit Types

• Aluminum oxide is the most commonly used sandpaper abrasive material, and is effective on wood, metal, paint, fiberglass and plastic. The particles fragment when exposed to heat and pressure, so new sharp edges are continually exposed. Garnet is used primarily for woodworking. It is not friable (does not fragment) and wears out faster but produces a shine on wood. Silicon carbide is harder and is best for metal, paint, plastic and fiberglass. Ceramic is the hardest of all common sandpaper abrasives. It only comes in large grit size and is used for shaping and leveling wood.
  
  

Grit Sizes

• Each grit size has an application. Use coarse grits (40 to 60) for stripping and heavy sanding. Medium grits (80 to 120) are designed for smoothing and removing smaller imperfections. 150- to 180-grit sandpaper is fine and should be used for the final sanding before finishing wood. Grit size 220 to 240 is made for sanding wood between coats of sealer or varnish. Extra fine (280 to 320) is used to remove dust spots between finish coats, while 360- to 600-grit should be used for the final finishing of wood.
  
  

Grades

• Sandpaper comes in two grades: commercial and industrial. Commercial-grade sandpaper is found in building supply stores and is made for consumer use in home projects. Its backing is made from Kraft paper or inexpensive fabric, and the grit is attached with hide glue. Commercial sandpaper is longer-lasting and designed for commercial production. It uses cotton or polyester fabric backing, higher-quality grits, and higher-grade resin bonding agents to keep the grit attached to the backing longer.
  
  

Open Coat or Closed Coat

• Open-coat sandpaper is recommended for woodworking.The grit is wider spaced to allow the sawdust to disperse and prevent clogging of wood. Closed-coat sandpaper has less space between grit particles and is not recommended for wood. When sanding, it is recommended to "go through the grits." This means starting with a coarse grit and working down through the various grit sizes. Changing directly from a coarse grit to a very fine grit could leave scratch marks.