There are a few ways to repair a rust hole, but no guarantees a repair will last. With that said, repairing a rust hole is easy and can be done by anyone willing to put forth the effort. Repairing a rust hole requires providing a strong repair that resists cracking. This can be accomplished best using a fibreglass cloth kit purchased at any automotive supply store. It doesn't matter if the rust hole is large or small; this system will work well.
Cut away any excess trim and loose debris from the area, leaving enough metal to maintain the shape of the body.
Grind away any trace of rust, primer and paint at least 4 inches back from the edge of the rust hole. Insert a 24-grit disc into a grinder.
Create a clear and shiny surface with a 100-grit sanding block. Use a ball-peen hammer to slightly tap the edges of the rust hole inward.
Cut a piece of release film, found in the kit, 3 inches larger than the sanded area around your hole. Lay the film over the rust hole and mark the sanded area with a pencil. Move it to the side.
Measure two pieces of fibreglass matt to cover the repair; one piece 1 inch smaller than the sanded area, and the second piece 1 inch smaller than the first. Lay them out on a flat surface.
Spread a layer of autobody repair jelly over the release film. Place the smaller piece of fibreglass on top of the film. Spread more repair jelly on the fibreglass cloth and place the larger fibreglass cloth on the smaller one.
Make sure the release film is facing outward and place the entire repair material over the rust hole. Smooth the material outward with a spreader removing all air bubbles and following the shape of the body.
Allow the repair material to cure overnight and then remove the release film. Sand the entire area with an electric drill and a sanding disc. Use a block sander to complete the job.
Fill any low spots with the filler material found in the kit. File down the filler after it dries and smooth it with an 80-grit sandpaper.
Add a smooth finish by applying a topcoat and hardener, found in the kit, with a spreader. Use a 200 and then a 400 grit sanding block to smooth the topcoat, preparing it for masking, priming and painting.
The ideal temperature for repairing a rust hole is between 60 and 90 degrees Fahrenheit. If the weather is hotter or colder, the repair material might need additional time to cure.
Wear goggles to protect your eyes when using power tools. Wear gloves to protect your hands.