Knowing what grit to start with and when to stop is the key to a perfect sanding job. Starting with too coarse a grit leaves deep scratches that are tedious to sand out. But starting with too fine a grit eats up time and paper, as does finishing up with a finer paper than is necessary for the job. Here are some guidelines:

Reading a Sheet
Time was you could read the back of a sheet of sandpaper like a book. Printed there were the grit size and type, the weight of the paper, the kind of glue used, and whether the sheet was open or closed coat. Now, about the only thing you'll consistently find, other than the manufacturer's logo, is the grit size. But even that bit of information can be confusing. Sandpapers in the U.S. have traditionally been graded according to the Standard Scale, in which the the numbers grow larger as the grits get finer. A "P" in front of the grit number designates a European system, which closely corresponds to Standard grades up to 240 grit, but diverges as the grits get finer. (A P800 is the equivalent of a Standard 400.)

A grit number followed by the Greek letter µ (pronounced "mew") indicates micron grading, in which the numbers get smaller as the grit gets finer. (A 60µ is about the same as a Standard 240.) These pricey "papers" — their backing is actually plastic film — are prized by auto-body refinishers and furniture makers.

Going Through the Grits
Sanding is a step-by-step process of scratching a surface with progressively finer abrasives. Each successively finer grit abrades away the marks left by the previous, coarser one, until the surface looks and feels smooth.

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