if the hole is rusted through(open)then u need to sand a area down to metal about 1 or 2 inches around the holes and grind the rust away so u can get a good adhesion.depending on how extensive u want to get theres a couple of ways,easiest depending on where the on the car it is u can just lay a little piece of fiberglass over it.spread resin on it and let it indent(concave) in the hole alittle bit and then put some body filler(bondo) in it sand it all down as smooth as ur little heart disires as long as u dont go past the body line then seal it up with some primmer and wal la.