From what I have read in a couple of places, flanging prevents some blow through welding but if not properly prepared where the metal meets is a breeding ground for rust. I`m going to weld some patch panels but I think I`m going to use these small clamps that hold the butt joint together the right amount of distance. The kit also comes with copper pieces to use as a backing so the welding won`t blow through.

1964 Chevy Big Window Fleetside