[bnoon]

What size wire are you running on your MIG? Flux core or gas shielded? Either way, I've noticed that sheet metal can be tough with thinner wire. If it's not a primer problem that is... Try some bare sheet metal once with it and see if the issue goes away, if not, bump the wire size up to the next size. If it is a primer problem, then a quick wire brush/sander/grinder next time.

With sheet metal though, it's best to do very short bursts and move around a lot anyway, so you probably did just fine.