I just stumbled upon this thread. I'm not sure where this question came from but here is my advice. NEVER use fiberglass unless you are patching fiberglass. Fiberglass will hold moisture and over time it will make more rust. Lots more. I'm still looking to kill the previous owner of my 65 fastback who thought it would be good to patch the entire inner trunk with fiberglass. I ended up having to replace all the metal from the rear seats back. Remove the rust that is there, weld in metal to repair any holes, work the metal until it is as smooth as possible, and then use a VERY THIN coat of putty to smooth it out. Good luck!!