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.