Sandpaper consists of a paper backing with an abrasive mineral coating. Inexpensive flint paper cuts slowly and dulls rapidly. Use it only for such jobs as removing old paint, where it will clog before the abrasive wears out. Garnet paper is more durable and is best for hand-finishing wood.
Aluminum oxide is a highly durable, all-around paper for hand or power sanding on wood, metal, or plastic. Silicon carbide, harder and sharper, does not last as well. Use it to smooth paint or varnish. If it has a wet-or backing, you can dampen it with water or oil for an extra smooth finish.
Sandpaper is graded by grit numbers. Use coarse (under 80) for shaping wood and removing heavy finishes. Medium (80 to 100) is for smoothing rough edges and cuts and for early sandings of wood and plaster patches. Use fine (120 to 220) for the final sanding before finishing and very fine (280 and up) for smoothing under-coats and polishing finishes. To avoid marring a surface, use no coarser a grade than necessary. Work in steps from coarse to finer grits.