The easiest way to sand plywood is to use 180-grit sandpaper to go over the surface. Once done, if the surface requires more sanding, change to finer-grit sandpaper as you do not want to sand away too much. Sanding is mostly required for lower-grade plywood products that have knots and other imperfections. If you are painting plywood, you will also have to do some sanding after applying filler and between layers of plywood primer and paint. The purpose is to get the surface to be as smooth and even as possible.