Rust

Hi, I have rust beneath my gas filler, I sanded it (by hand 60 grit) out because im trying to strip it to bare metal. And found 2 holes on both corners from the bottom of the gas filler door and feel a rubbery film. I checked the filler stem itself underneath and it seems fine. So could I possibly sand all the rust off, and possibly put fiberglass to fill the holes, sand it down again with 150, and go over it with tremclad. Im not looking for a professional.I respect body guys for my 04 gto. I just want it solved as easily as possible as this is a beater for many people to come, good car with the 2.2. Ps. The tremclad is pretty much the same colour. Dark blue.