Painting a metal or fiberglass surface, like those found in an auto body, often requires that you deal with flaws in that surface. Dealing with light scratches or gentle rises and falls in the surface tends to require only a bit of applied sanding. Dealing with major flaws like gouges, rust or dents often requires a bit more effort, including the application of a filler material like Bondo.
Clean the application surface with warm, soapy water using a sponge. Wash it thoroughly to remove any traces of wax oil or dirt. Pat the surface dry with a clean cloth.
Smooth the surface with 80-grit sandpaper in a small, circular motion until you've removed all traces of the covering material. Remove any sanding residue with a wax-impregnated tack cloth.
Knead the tube of hardener to soften it. Place a golf-ball-size chunk of Bondo onto a piece of scrap plywood. Squeeze about a centimeter-long string of hardener onto the Bondo. Stir the mixture. Keep the mixing ratio consistent, according to weight of the material you use. Use a mixing ratio of two parts hardener to 100 parts putty by weight.
Spread the mixture over the repair area. Press firmly on it to make sure it covers every inch of the surface. Allow it to dry for about 20 minutes.
Sand the Bondo smooth with a power sander to rough out the final shape. Switch to handheld sandpaper to finish shaping the putty with more precision. Prime and paint the Bondo as needed to match the surrounding surface.
