In order to sand and repair drywall, you will need the following: 100 grit sanding sponge. After you have applied mud to the walls evenly, you will need to sand the walls until they are smooth. Hopefully, you minimized or mitigated most of the need for sanding in the application of the mud, by using feathering and thin layering. When the mud is completely dry, you will need to choose a good sander. It is recommended that you use a 100 grit sanding sponge. Place the sponge against the wall, and rub it up and down until it is smooth. The sanding sponge will allow you to sand the corners more easily. Sand the entire drywall surface.