You can apply the filler in layers-thinner layers dry faster and are easier to sand by hand. Start knocking the filler down by using a coarse-grit paper, and the closer you get to your desired surface height use a finer grade. Don't sand the body filler by hand as your work will end up uneven. Make sure you use a sanding board or a sanding block for smaller areas. Once you are happy with your work, clean the surface and spread a quality glazing and spot putty over the filler. the glaze will fill pinholes and tiny imperfections. Let the glaze dry thoroughly, and once it is fully cured sand it down with 400-grit sandpaper. After you're satisfied with your bodywork, prime the entire surface of your project. Once the primer is dry it should be lightly sanded with 400-grit paper.