The development of two important synthetic abrasive materials, Aluminum Oxide and Silicon Carbide, occurred in the early part of the 20th century. Synthetic adhesives began replacing animal glues and varnishes by 1935. Polyester and other heavy duty backings began to be used in the 1960's. The coated abrasive industry has seen immense advances in the past twenty years with breakthroughs in grains, bonding, backings and splices as well as in the machinery that uses abrasives.

There are three primary ingredients to modern sandpaper - Abrasive Grains, the Backing, and an Adhesive.

Abrasive Grains

The abrasive grains used in sandpaper today are either natural or synthetic minerals that are crushed, then cleaned and graded for size. Garnet is a naturally occurring mineral which is still widely used in the manufacture of sandpaper. It is useful for producing a nice smooth finish on wood, but lacks the toughness needed for most sanding needs.

Abrasive grains obtained from synthetic minerals such as Aluminum Oxide, Silicon Carbide and Alumina Zirconia are used in most of the sandpaper available today. These synthetic minerals are made by combining natural substances such as Bauxite clay and coke, from burning coal, in a large furnace and heating them to extremely high temperatures. The heating and specialized cooling process can take days, and the mineral that results has a crystalline structure that cuts faster and resists wear better than any naturally occurring mineral.

What the manufacturers are looking for when they produce these synthetic minerals is an abrasive grain that is hard and tough, two very different properties. Silicon Carbide is very hard, only slightly less hard than diamond, but it lacks toughness. The particles break down under use, fracturing and producing new cutting edges. This is a useful aspect for some uses, such as smooth sanding paint and fine furniture, but other uses require tougher grains. Aluminum Oxide is not as hard as Silicon Carbide, but it is tougher, or less "friable," producing abrasive grains that wear much longer under hard use.

Backings

Paper has been the traditional backing used in the manufacture of sandpaper, and is still used today. The paper used can be made thick or thin, heavy or light, and can have chemicals or other materials included that make it more flexible or resistant to moisture. Paper generally has a smooth surface and is relatively stiff as backings go; it will produce a smoother finish that the same grit abrasive with a different backing.

Strength and durability can be a problem with paper. The coated abrasive industry has therefore turned to cloth to provide a tough, durable backing for their abrasive grains. Traditionally cotton cloth has been used in a number of different weights. More recently polyester and cotton/polyester blends have gained wider acceptance. Cloth does not have as smooth a surface as paper, and is a stiffer material, and so provides a more aggressive cutting action and leaves a rougher surface. Most sanding belts and discs are made with a cloth backing for durability.

Vulcanized fibre is a backing often used for heavy duty sanding discs to provide an extremely stiff and strong support for the abrasive grains. Backings made from films of polyester and other synthetic materials have been developed in recent years for use in various high-tech industries. They provide a surface that is extremely smooth, flat and durable. Sandpaper made with these backings are quite expensive, but result in a very fine, consistent finish.

Adhesives

Without adhesives sandpaper would only be grains and paper. Prior to the 1940s the traditional adhesive used to make sandpaper was animal hide glue. Varnish was also used as an adhesive to make a waterproof sandpaper. Glue still has some limited use in sandpaper, but resins have taken over. Urea-formaldehyde resin was the first modern resin to be used as a bonding agent in the coated abrasive industry. Today most sandpaper is made with a phenolic resin. These resins are strong, heat resistant, and for the most part water resistant.

Sandpaper requires two coats of adhesive. The first is the "make" coat. This is the layer of adhesive applied to the backing that adheres the abrasive grain. The second is the "size" coat. This layer of adhesive is applied after the abrasive grain is attached, covering the entire surface. The size coat locks the abrasive onto the surface of the sandpaper. Usually the same type of adhesive is used for both coats, but there are a few manufacturers that produce a resin-over-glue that offers many of the best qualities of both types of adhesive.

Standards and Grit Size

CAMI   FEPA   Particle size US Standard   P Scale   in Microns           1200       5 1000       9 600   1200   15 400   800   20 320   500   30 280   360   40 240   320   45 220   220   60 120   150   100 80   100   150 60   80   190 40   40   410 16   20   980

Abrasive minerals are crushed from larger into smaller pieces, then are screened and graded based on the size of the grain, called grit. The larger grains can be sifted out using screens made of fine silk threads. The finer grits, above 240, are separated by blowing through the air, centrifugal methods or by sedimentation in water.

The standard scale for grit size in the United States is established by the Coated Abrasive Manufacturer's Institute (CAMI). In Europe the standards is set by the Federation of European Producers of Abrasives (FEPA). Be aware that there are some differences between these two standards. They are similar for the larger grit sizes, up to about 220, but differ widely for the finer grits. If the FEPA standard is used, the grit number will have a "P" in front of it.

In general, finer particles have higher grit numbers. See the chart for a comparison of grit sizes.

Choosing The Right Sandpaper For The Job

The proper approach to choosing sandpaper is to know the job it is intended for, and then determine which qualities meet your needs. The obvious starting point is to determine which grit size you need. A 60 or 80 grit is fairly aggressive, 160 to 180 is considered medium, and above that is a fine grit. If you need to get down to a level surface or remove a coat of paint, 80 grit is a good place to start. If you are sanding to obtain a smooth finish on wood, start with a heavier grit, then move up in stages, for example 80, then 120, 180, 220 and finish with about 280. Each step up in grit size will remove the scratches left from the previous grit. If you skip a grit, it takes longer to remove the scratches.

In terms of abrasives, most sandpaper sold today uses Aluminum Oxide grain, and it is a good choice for most uses. It produces a long lasting, even cutting sandpaper. Garnet wears out faster, but produces a softer finish on wood, which is particularly handy if the wood is to be stained. Silicon Carbide is often used in paper produced for wet sanding, and is very good for the final finish sanding.

Your choice of backing material is as important as your choice of abrasive. If you are trying for a smooth finish by hand sanding, paper is a good choice. If you need rapid stock removal or other heavy-duty uses, cloth is a better choice. Most sanding belts are made with cloth backing.

The adhesive used is also important, but there are fewer choices. If the sanding job will produce much heat, stick with a resin bond, as glue is heat sensitive. The amount of adhesive applied to the sandpaper can also make a difference. If the backing is densely coated, which is the usual method, it is called a "closed" coat. These give an even finish. If you need to have rapid stock removal, look for an "open" coat. These have only 50% to 75% of the surface covered with abrasive grains. This reduces "loading", a condition where the particles removed from the sanded surface eventually coat the sandpaper and eliminate the cutting action.

All of these features of the sandpaper should be clearly marked on the package of sandpaper. They are also marked on the sandpaper itself, but may be in the form of a sequence of numbers and letters, more of a code than useful information. For most uses I recommend a good quality Aluminum Oxide on a cloth backing with a resin-on-resin adhesive. Use your knowledge of the particular sanding project to guide your choice of sandpaper from here.

Reprinted by permission of: Scott Greaves