Yeah I painted rust inhibitor on it after wire wheeling and sanding it heavily. There are drain plugs back there. It wasn’t noticeable at all until the bumper was removed.Did you spray the inside of that fender with anything to stop the rust that was already there? That's such a tricky spot for rust to hide especially when you have the stock bumper. Are there any drain plugs back there? I don't remember there being any. Fiberglass works great!