Due to improvements in the steel used to make cars, the problem of rusted body panels is greatly reduced from several years ago. In some parts of the country, however, rusting is still a serious problem, and in just a few years a hole can develop in your pride and joy. It is, of course, best to fix the rust before it becomes a hole, but with some effort, you can fix a hole in a car with Bondo to make it look as good as new.
Remove any metal that is not solid from around the area of the hole that is to be repaired.
Use coarse-grit sandpaper to remove any paint and primer for a distance of 3 to 4 inches in all directions away from the hole. For the best and longest-lasting repair, all the rust should also be removed. It is likely that some small rust pits will remain, however.
Use nylon window screen to fill in behind the hole and create a backer to hold the auto body filler. Make sure that none of the screening is protruding from the hole. The screen can be bunched up and pushed through the hole in areas where access from the rear is impossible. In those areas where the back of the panel is open, a piece of screen an inch or two bigger than the hole can be held in with tape, epoxy or strong adhesive.
Mix the amount of body filler that you can use in 10 minutes following the mixing directions on your can of body filler. Generally this is a ribbon of hardener about 2 inches long to a golf ball- sized glob of filler. Use a piece of cardboard as a mixing pallet. Mix well with a small plastic spreader until the colour is uniform. Try to not mix air into the filler since this will create pinholes in your repair.
Push filler into the hole, forcing it into the nylon screening. Use a plastic spreader and completely cover the area of the hole. Feather the edges of the body filler away from the repair. Putting extra effort in to getting the filler spread evenly will pay off here in reduced sanding time.
Allow the filler to harden to the point when it can be dented with a fingernail. Remove excess filler from the repair with a filler file (sometimes called a "cheese grater"), if necessary. This type of file is very ineffective once the filler has truly hardened, so work quickly.
Start with coarse sandpaper and sand the repaired spot smooth. Progress through medium and fine grits to make the filler as smooth as possible.
Fill any deep gouges that remain with more filler, sanding when it is hard, to finish this phase of the repair.
Spray the repaired area with sandable automotive primer.
Allow the primer to dry completely, and sand smooth with fine sandpaper.
Check for any small holes or gouges and fill them with spot putty. Wait 24 hours or so, then sand the puttied areas with fine sandpaper. Spray-prime the area again and allow to dry.
Wet-sand the repaired area with 600-grit wet/dry sandpaper. Rinse any sanding dust away completely.
Allow the area to dry completely, and spray with the colour-matching finish paint. Spray at least two coats, but avoid causing a run from too much paint.
Allow plenty of time for this repair. The actual work does not take long, but allowing extra time for curing and drying will make your repair look professional. Don't rush.
When using body filler and paint, always work in a well-ventilated area away from open flames.