Using fibreglass to repair a rust hole is one method of repair that is both inexpensive and effective. To start the repair, you need a fibreglass repair kit. This costs about £16 as of 2011, and it includes all the items you need to make the repair: fibreglass sheets, wire mesh screen, application disc, resin and hardener. The repair is used on sheet metal, roofing, even appliances. It is also a solution for car rust holes in floor boards or the boot.
Remove all the rusted metal from around the hole by cutting it away with metal sheers or a metal saw. Sand the metal with sandpaper or a sanding disc and grinder until only non-rusted metal remains around the hole.
Lay the wire mesh screen over the hole and cut the screen to fit over the hole with about an inch of overlap onto the metal surface. Lay fibreglass sheets over the hole, cutting the sheets to extend past the hole and screen, or about two inches.
Mix the resin and hardener following the mixing ratios on the instructions in your fibreglass repair kit closely. Hardeners are normally mixed in small ratios to the fibreglass resin compound, but consider your humidity level when mixing.
Lay the mesh and fibreglass over the hole and apply the resin over the strips. Or, soak the strips in the resin and apply them over the screen, whichever is easier for you. Allow the compound to harden and set for 12 to 24 hours or until completely dry. Drying time varies based on humidity and the ratio of hardener to fibreglass used during mixing.
Sand the finished repair with 200-grit sandpaper. Apply a metal primer, let it dry and paint on the top coat to the repaired area.