| Abrasives: Which one should I use? |

Thankfully, most woodworkers don't need to do extensive testing to determine the best abrasive for their use. Keeping just a few basic facts in mind makes choosing an abrasive product relatively easy for most woodworkers.

Abrasive Materials

Of primary importance is selection of the abrasive grain. Four different materials are commonly used: Garnet, Aluminum Oxide, Silicon Carbide, and Alumina-Zirconia.

Garnet	Golden orange colored sandpaper that has been a woodworking favorite for years. It is a natural mineral that cuts fast, but wears out rather quickly. It is a good choice when sanding woods that tend to "load" up the paper before the grit has lost its bite. It has been largely replaced by Aluminum Oxide.
Aluminum Oxide	Familiar brown or reddish brown sandpaper that has become the standard for production woodworking. It is a synthetic abrasive which is very hard and tough, providing good abrasive life for both metal and woodworking applications.
Silicon Carbide	Shiny black man-made abrasive most often used for "wet-or-dry" sanding products. Its grains are very hard and sharp, but lack the toughness of Aluminum Oxide. It is long-lasting because the abrasive grains fracture as they wear, exposing sharp new edges. It is most often used as a finish sanding paper.
Alumina-Zirconia	Blue colored abrasive. It is also man-made, and is extremely hard, sharp, and tough. It is used chiefly in abrasive planing operations, where its superior life offsets the higher cost.

Backing Materials

Obviously, the abrasives are almost useless unless they are bonded to something, and that is where the backing comes in.

Paper Backings	Generally provide a finer finish than cloth backings, although they are less durable. Paper flexibility is governed by its weight. Lightweight papers are more flexible. Finishing paper is designated as "A" or "B" weight while cabinet or general-purpose papers are designated as "C" or "D" weight. Heavy-weight "E" and "F" papers are used for sanding belts.
Cloth Backings	More durable than paper, and find primary use for belts or disks. Lightweight belts are designated as "J" weight, and are suited to contour sanding. The most common backing is "X" weight; it is available on virtually all product types. Heavy-duty applications, such as abrasive planing, require the strength of "Y" weight backings.

Open vs. Closed Coat

One other factor enters into abrasive selection: Open or Closed coat bond.

"Open"	Refers to the fact that the abrasive grains are spaced farther apart to allow for the buildup of wood dust which, if heavy enough, can clog the paper. It is the most common abrasive style.
"Closed"	Have almost complete abrasive coverage, so that each abrasive grain abuts the next. This leaves very little room for sanding particles. "Closed" coat bonding is provides a very uniform scratch pattern, and is used primarily for fine grit products.

Reference: Klingspor Coated Abrasives Reference Manual