If you don't back it up with something it probably won't hold. If you can't weld or rivet it, you can use the jb to "weld" a small square of metal inside and let it cure. Then finish filling the hole from the outside and sand. Finish it up with some regular bondo so you can use finer sand paper to get the scratches out. Before you paint it seal it with some kind of good sealing primer. The idea is to use the least amount of jb as possible and use the metal square in the back for the strenght. Good luck with it.