It is relatively simple to prevent and repair holes in car bodies. More often than not, they are caused by the development of rust in the body of the car. If rust is allowed to set in, the rust can eat away at the metal of the car and holes can develop which can enlarge if not fixed promptly. Simple regular washing and immediate repair of small rust spots can prevent larger holes in future. It is relatively simple however to fix holes using an auto body filler like Bondo.
Using a putty knife or similar instrument, remove any loose rusted metal from around the hole that you are going to repair.
Use the coarse grit sand paper to sand the area and remove as much rust as possible. Sand the area around the hole in all directions up to at least 3 inches away from the hole.
Create a backing for the filler to adhere to using nylon window screen. If you can gain access to the area behind the hole, secure the backing using adhesive. If you can not access the back of the hole, squeeze some of the nylon window screen through the hole being sure that none of it is protruding out of the hole.
Using a piece of cardboard as a pallet, mix a small amount of filler using a putty knife. Work quickly as it hardens fast and do not allow air to mix in with the filler as this may lead to tiny air holes in your finished work.
Use a plastic spreader to spread the filler over the hole. Push the filler inside the hole and onto the nylon. Cover the whole area completely and smooth it out as much as possible using the spreader.
Monitor the filler while it hardens. Press your finger nail into the filler at various intervals. When the filler is almost dry, but you can still dent it with your finger nail, use what is known as a "cheese grater" or metal file to file away the excess. Do not allow it to fully harden before you do this otherwise the metal file will be ineffective.
Sand the filled in area using coarse grit sand paper by hand. switch to a finer grit and finish off with very fine grit sand paper. Continue to sand the area until the area is as smooth as possible.
Evenly spray the area with auto primer. Let the primer dry completely according to the manufacturers instructions. Sand the area again with the fine grit sand paper.
Use spot putty to fill in any remaining holes. Sand again when dry and allow the spot putty to dry before spray priming and sanding the area again with the fine grit sand paper.
Use 600 grit wet/dry sand paper to wet sand the area. Rinse the area thoroughly with water to get rid of any grit and sand on the surface of the repaired area.
Leave the area to dry thoroughly. Use your matching colour auto paint to paint over the area in up to 3 thin layers. Allow each coat to dry before applying the next coat.
Do not rush. Allow plenty of time for the filler and paint to dry before moving on to the next stage.
Use paint and filler in well ventilated areas. Work away from naked flames.