Sandpaper is a backing sheet covered with glass, aluminum oxide, silicon carbide, garnet or other special sanding granules called grit. Sandpaper is sold in many forms including sheets, rolls, discs, belts, and special pre-cut shapes for use in commercial sanding machines. Sandpaper sold for use with power tools is generally heavier and more durable than that sold for hand sanding projects. The correct sandpaper is determined by the abrasiveness of the grit, the adhesive to the backing and the type and size of the grit. It is also referred to as both “open coat” and “closed coat” which refers to how far apart the sanding granules are placed on the paper. Open coat is needed for jobs like removing paint or working with resinous woods that leave material that clog the sandpaper. Something like dry wood that does not clog would get smooth results using a closed coat paper.
For best results use the correct type and grade of sandpaper for the sanding job you are attempting. In other words, don’t use paper suitable for hand sanding when using a power tool. Begin with a medium to course grade of paper and change the grade as the job progresses. Always sand with the grain of the wood. When sanding with a lubricant, do not let the surface dry out. Sandpaper should be stored in a cool, dry area as dampness may cause the backing or adhesive to fail and the grit to become detached.
Grit:
The size of the grit is referred to by a number: The lower the number, the courser the grade of grit. A universal grit chart follows:
Grit Size Grade
40-60 Course
80-100 Medium Course
120-150 Medium
180-200 Fine
240+ Very Fine
Sandpaper sheets are normally marked on the back with the numbered grit size and/or the grade.
Glass grit sandpaper is made of quartz granules on a paper backing. Glass grit is a soft abrasive used for sanding painted or natural wood, metal and other materials.
Aluminum oxide sandpaper is used for shaping, sanding and polishing metal such as steel and iron but can also be used on wood. Aluminum oxide has a longer life than glass or garnet sandpapers. Aluminum oxide is available in ordinary paper, waterproof and cloth backing.
Garnet, which looks like sand, is the most common and most economical sandpaper.
Garnet sandpaper works wonderfully on general wood sanding, smoothing hardwoods and for fine finishing work. Garnet grit lasts longer than glass but not as long as aluminum oxide.
Silicon carbide sandpaper is suitable for both wet and dry sanding. It can be used successfully on hardwood, brass, aluminum and plastic. It can also be used for smoothing cut glass edges. Silicon carbide is almost as hard as a diamond but it is brittle and will wear down fast when used on hard metal.
Backing materials:
The backing materials are as important as the type of grit. The most common three types available are ordinary paper, waterproof paper or cloth.
Ordinary paper: Generally adequate material for most sandpaper. Power tools generally require sandpaper with a tougher quality of backing material than that for hand use.
Waterproof paper: For use with a lubricant. The back is usually darker and glossier in appearance.
Cloth: Used when a high degree of flexibility if necessary.
Backing adhesives:
Ordinary paper sandpaper uses a water-soluble adhesive. Waterproof sandpaper, which is designed to be used with a lubricant, uses a waterproof adhesive. Regardless of the type backing adhesive used, the important thing is that the bond between the grit and the backing material do not separate when used.
