Sandpaper Primer

Sandpaper has been around for a very long time and the types of materials used to make sandpaper has varied greatly during that time. For example, in 13th century China, sandpaper was made out of crushed shells, seeds, and sand. Later on, sandpaper was made with crushed glass and was known as glass paper. Flint was also used for many years. Today, most sandpaper is made with a cloth or paper backing that is coated with either a man-made or natural abrasive.

Choosing the appropriate sandpaper for a woodworking project involves a basic understanding of sandpaper abrasives, sandpaper grits, and to a somewhat lesser extent, backing materials. Once you have a basic grasp of these concepts, you won't be befuddled when the time comes to place an order for sandpaper. Of course, sanding technique is also important but that's the topic for another article.

Sandpaper Abrasives

The type of abrasive plays a key role in how long sandpaper will last, how quickly it cuts, the types of materials it works best on, the smoothness of the sanded surface, and the cost of the sandpaper.  Here is a brief overview of the primary abrasives used for sandpaper:

Aluminum Oxide

• Most common sandpaper abrasive for woodworking.
• Lasts longer than garnet and silicon carbide sandpapers.
• Works well on bare wood as well as painted surfaces and metal.
• High degree of friability but not always the case with cheaper grades.
• Typically brown in color.

Garnet

• Natural mineral with a distinctive orange color.
• Wears faster and does not cut as quickly as aluminum oxide.
• But has finer scratch pattern making garnet a good choice for final sanding.
• Some woodworkers like to start with aluminum oxide and finish with garnet.
• Has tendency to burnish the wood surface allowing for more even stain penetration.

Silicon Carbide

• Sharper and harder than most forms of aluminum oxide but the grains tends to shear off while sanding wood, causing the abrasive to wear down quickly.
• Usually black (wet/dry) or gray (stearated) in color.
• Can be used wet or dry.
• Good choice for smoothing finishes between coats and for sanding metal.

Ceramic / Alumina Zirconia

• Tough, sharp, and long-wearing.
• More expensive than other types of abrasives.
• Typically used on sanding belts and disks, sometimes blended with other abrasives.
• Usually brown; sanding belts are blue.

*Friability is the tendency of an abrasive to fracture with wear and expose new, sharp cutting edges. This is considered a good thing because it extends the life of the sandpaper and provides a more consistent scratch pattern.

Not all sandpapers are made the same. There are variations between brands and even within brands for a given abrasive type. For example, silicon carbide sandpaper is generally considered best for sanding finishes rather than bare wood. In my woodshop, I have used silicon carbide sandpaper from Mirka for a number of years and have good results sanding both wood and finishes with it. Other brands that I have tried did not work so well with bare wood. Another thing I like about the Mirka sandpaper is that it has a fairly durable backing that tends to hold up better than some cheaper products.

Open Versus Closed Coat

Sandpaper can be categorized as open-coat or closed-coat based on the density of the abrasive particles. With closed-coat paper, the abrasive particles cover 100% of the surface. With open-coat paper, the particles cover 40% to 70% of the surface. Open-coat sandpaper is best for wood because it doesn’t clog as easily. Closed-coat is better for sanding metal and wood finishes.

Stearated Sandpaper

Also known as no-load or self-lubricating, stearated sandpaper (usually made with silicon carbide) contains metallic soap to lubricate the sandpaper in order to minimize clogging and extend the useful life of the sandpaper. This lubrication is especially beneficial for sanding finish coats since ordinary paper would quickly clog, rendering it useless.

Sandpaper Grit

Sandpaper is rated by the coarseness of the abrasive particles. This rating is known as the grit. It is important to choose the proper grit because it determines how aggressive the paper is at removing material as well as the smoothness of the sanded surface. Grit ranges from 16 to 1=2000 (CAMI scale), although grits above 600 are seldom used for woodworking applications. The table below summarizes the most common sandpaper grits. Note that the higher the grit, the finer the sandpaper.

 

Grit	Common size	Uses
24-36	Extra Coarse	Very fast material removal for stripping thick finish or shaping wood
40-60	Coarse	Fast material removal  for stripping finish or shaping wood
80-120	Medium	Removal of saw marks and other surface imperfections
150-180	Fine	Final sanding of bare wood before applying finish
220-240	Very Fine	Sanding after initial finish coat
280-320	Extra Fine	Sanding between finish coats
360-600	Super Fine	Final sanding of finish

The grit numbers in the table are based on a scale developed by the Coated Abrasives Manufacturers’ Institute or CAMI for short. The CAMI standard is commonly used for sandpaper sold in the United States. Another standard known as Federation of European Producers of Abrasives (FEPA) is also widely used, especially in Europe. If the back of your sandpaper has a “P” prefix in front of the grit number, like P120, then it is graded according to the FEPA standard. Otherwise, it is probably the CAMI standard.

The two grading standards are roughly comparable for coarse to medium-grit paper. For example, paper graded as 120 CAMI is about the same as that graded as P120 FEPA. Once you get above 220 or so, the two systems diverge and FEPA graded sandpaper becomes increasingly coarser than CAMI graded paper for a given grit size. The important thing to remember is that if you’re doing fine sanding, be aware of what grading standard applies to your sandpaper.

Sandpaper Backing

Sandpaper is made by gluing abrasive materials to a backing. This backing may be paper, cloth, fiber, or a combination. For paper backings, the thickness of the backing is denoted by the letters A to E from thinnest to thickest. The A and B weights are the most flexible and used for very fine grits. The most commonly used weights are C and D. These are the ones to use with your palm sander. E and F are used for sanding discs and belts. There are also J and X weights for cotton cloth backing. J is used for sanding curved surfaces and X is used for heavy-duty sanding belts.

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