Coated abrasive sandpaper is separated into different grades. The Federation of European Producers of Abrasives and the Coated Abrasives Manufacturer's Institute define the grades by grain size. Grains of sandpaper are measured in microns; a micron is 1/1,000,000 of a meter. Sandpaper grades are categorised as very coarse, coarse, medium, fine, very fine, extra fine, or super fine. The grading system helps consumers decide what they need for individual projects. Coarse sandpaper is for rough sanding, and the fine grades are for finishing projects.
Use very-coarse-grade sandpaper to remove thick rust build-up and multiple layers of paint and to sand rough-cut wood. Average particle size is 1,842 to 1,324 microns. Use this grade to strip a surface quickly and leave a rough surface.
Remove light rust using coarse-grade sandpaper. This grade of sand paper works well for removing a single coat of paint or sanding rough wood. The average particle size is 638 to 336 microns. Using this grade leaves a surface smoother than does using very-coarse sandpaper, but the surface will still be rough.
Finish a surface by using medium-grade sandpaper. After you sand lumber with this sandpaper, you can paint it. The average particle size is 268 to 141 microns. This grade of sandpaper leaves a surface smooth.
Create a lightly polished finished surface with fine-grade sandpaper. The average particle size is between 125 and 82 microns. Sand with this grade of sandpaper before varnishing or lacquering wood.
Create a polished finished surface on metals with very-fine-grade sandpaper. The average particle size is 44.0 to 58.5 microns. Use this grade to sand between painted layers when applying several coats of paint. This helps the paint layers bond.
Remove imperfections from handmade jewellery or unfinished porcelain projects with extra-fine sandpaper. The average particle size is 36.0 to 25.8 microns. You can also use this sandpaper grade to sand woodgrain raised by stain.
Use super-fine-grade sandpaper to create a very highly polished finished surface on a car's paint finish or to sand polymer clay projects. The average particle size is 19.7 to 6.5 microns. This grade of sandpaper is the finest available; use it to create a glasslike surface.
When sanding metals, use a circular motion; when sanding wood, use a back-and-forth motion in the direction of the grain.