Using Plastic Body Filler - Part II
After you have properly mixed your body filler and hardener, apply it in the following manner.
First, apply a thin initial coat to the part using stiff pressure on the blade. Flex the blade so that it is nearly parallel to the panel. This helps eliminate air from being trapped in depressions in the metal surface.
After the initial thin first coat, apply more filler to build up the surface as needed. After you have applied enough filler, use a "cheese grater"-type plane to rough-finish the material. Use this tool just after the filler has hardened enough to permit working with it. If you wait too long, the cheese grater will not cut the filler and you will have to use a grinder or file. Tip: it is much easier to plane the material than to file or grind it, so stay alert and use the cheese grater plane. This tool is also more effective for revealing low spots and leveling high spots.
The cheese grate tool will leave a corrugated, rough surface that you will need to plane off. But first check for low spots and apply more filler as needed.
Clean the tools you used for mixing and application immediately after use. If filler has hardened on the tool, thinner will take clean it off. If you are doing a lot of filler work, have a can of thinner in which you can place your tools so that nothing hardens and dries while you work.
After you have filled in low spots, plane the high spots to remove the ridges left by the cheese grater planer. Then apply a finish coat to the entire surface. Applying a finish coat eliminates pinholes and tiny pits. If you have properly applied then planed down the first coats, applying the finish coat should be easy.
Sand the finish coat using rough paper; use a finer paper for the final finish. The procedure is the same - going from rough to fine paper - whether you sand with a machine or by hand.
The surface has the proper contour and smoothness because you have previously shaped and straightened the metal. It is now ready for priming and painting.