I am fixing holes and gouges. It seems the previous owner only had a can opener for tools. I spot weld with my mig on the outside and bead the inside. when I grind down the outside, there are small craters or high spots. My question is.... What do I use to smoothe it all out? Bondo, Eastwoods metal to metal,etc. This area is the underside. Tunnel hump,Floor area,fire wall. Before I seal and paint, I think I should use something easy to work with , but durable and some heat resistant.