Although the technique of using sand as an abrasive goes back to the stone age, the first recorded example comes from the 13th century, when Chinese craftsmen bonded sand, crushed shells and sharp seeds onto parchment with natural gum. In the western world, abrasive paper was being used in France by the 18th century and glasspaper, which consisted of glass particles glued to paper, was mass-produced In England by the 1830s. The U.S. Patent Office issued a patent for sandpaper at about the same time.
Today those shells, seeds, glass and even sand have been largely replaced with synthetic abrasives, primarily aluminum oxide and silicon carbide, and sometimes by the minerals garnet and emery. Emery, which is black, is an impure granular form of corundum, a mineral that, in its pure form, is second in hardness only to diamonds. When it is crushed, emery isnt very sharp, which makes it better for cleaning and polishing metal surfaces than for conventional sanding projects.
Garnet is sharp, but not very hard, which means it wears out quickly on a hard surface. Garnet paper has a distinctive reddish color; woodworkers love it because it produces a very smooth surface finish and tends to burnish wood. This is desirable when using pigmented stains. But neither garnet nor emery are good for most sanding jobs on a boat.
You might also come across flint paperactually its quartzand typically its color is light gray or beige. Flint paper is cheap, but you get what you pay for; quartz is neither hard nor sharp.
Aluminum oxide is the most common sandpaper abrasive. Fused aluminum oxide is essentially synthetic corundum, but is purer than emery. Because it is brittle, aluminum oxide paper creates fresh cutting edges while the paper is being used. Thats why it tends to last longer than other sandpapers. Typically tan or brown in color, aluminum oxide paper is the best choice for most marine refinishing projects.
Silicon carbide, or carborundum, has a black surface and is commonly known as wet or dry sandpaper. Wet or dry is a trademark of 3M, the company that originated waterproof sandpaper. There is also open-coat silicon carbide paper, which is white. Because silicon carbide is harder than aluminum oxide, its the best paper to use when sanding hard materials like fiberglass, paint and metal. Its not good for sanding most woods or other soft surfaces.
What paper to use?
For wood and most of the other surfaces on a boat, the best choice, even for power sanding, is aluminum oxide. For wet sanding, metal sanding, between-coat sanding (or if youre involved in a gelcoat restoration), use waterproof silicon carbide paper instead.
Every piece of sandpaper has a grit number that describes the particles of abrasive per square inch of paper. The higher the number, the smaller the particles and the finer the finish you can expect. Grit numbers run from 36 to 1000, although there are some papers that exceed this range at either end. Cheaper papers are labeled coarse, medium and fine, which generally translates to 50-, 100- and 150-grit paper. Whenever possible, avoid sandpapers that do not have a numeric grit designation.
Most grit numbers also have a letter describing the weight of the backing paper; the most common designations are A, C, D and E. A is the lightest and least durable. When you have a choice, use paper designated C or D.