Mate it really depends,if you have NO access to a welder at all and bog is your only possible option go teh fiberglass..Make sure you get the better weaved matting and dont used too much hardener as it will have an opposite effect and stay wet for ages..I did heaps of work with the stuff on the red celica and it has come up well...As for por15 i think it says you can spray over the top of rust with that shit,its that good!