Ima third on Carl. There's been discussion in some circles that the heat filler generates, then cools, tends to create moisture. It'll get trapped between the filler and metal, causing rust issues until the filler falls off or loses its bond with the metal. Current practices are to seal with epoxy primer, then work your filler on top of that. You can re-epoxy, fill primer, seal or whatever method you choose after that, before you topcoat it.
