If you have let the rust go for too long on your car, you may notice that it has eaten all the way through the body. This is particularly likely to happen on the rocker panels underneath the doors and in the lower areas just behind the back wheels, where the body gets hit with a lot of slush and road salt. Rust holes should be fixed before they get even larger, potentially making the car unsafe.
Cut away all the rusty parts using tin snips or an electric metal saw. If you leave any rust present, it will continue rusting after your repair.
Cut back far enough that there is only shiny-looking metal remaining.
Cut the holes so that they are evenly sized to the extent that this is possible. A symmetrical hole will be easier to patch.
Cut a piece of tin to a size that is several inches larger than the hole that you have cut.
Bend the tin to match whatever profile is present in the car body at the point where you cut the hole.
Rivet the piece of tin onto the car body by applying rivets through the tin and the body, putting rivets every four to six inches.
Mix up a small batch of auto body filler. Auto body filler consists of a putty-like substance and a separately packaged catalyst. As soon as you mix the two, the mixture will begin to harden, so only mix as much as you can use within a few minutes.
Apply the auto body filler to the seam between the car body and the tin patch that you riveted on. Smear the filler in between the edge of the patch and the body of the vehicle, and spread it over the edge of the patch so that it is completely covered.
Sand the filler down after it has hardened.
Apply primer over the entire area. After the primer has dried, paint over the area using automotive paint.