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Sandpaper isn’t made of sand, of course, but of fine grains of the natural mineral garnet, or of synthetics like silicone carbide, aluminum oxide, or alumina-zirconia (zirconium). The ground-up materials are sifted though graded screens. The smallest particles are used in making the finest papers, the coarsest grains for coarse “coated abrasives,” as sandpaper is technically known today. The size of the grit is identified by a number.
The fine, medium, and coarse papers (there are, by the way, more precise gradations within these three main subdivisions) are used for different kinds of tasks. The standard grades of papers are these:
Extrafine. This grade of sandpaper is used between coats of paint or varnish. Grits of 240, 320 and 400 are termed very fine, while extra- or superfine sheets with grits of up to 600 are available for polishing jobs.
Fine. Fine abrasive papers have a grit in the range of 120 to 220. For most home workshops, fine will suffice for final sanding before the work is finished.
Medium. Some final shaping can be done with medium, which has a grit range of 60 to 100. General sanding work is often best done with medium-grade sandpaper.
Coarse. Rough shaping is the strong suit of coarse paper, as is the removal of previous finishes. The grits are typically in the 40 to 50 range.
Extra coarse. This stuff is really rough, usable for removing paint and varnish that you think might never come off. The sanding of old floors too, sometimes requires the abrasiveness of extracoarse sandpaper. Don’t even think about using it on any but the toughest jobs.