Get it mechanically clean with a stainless brush and make sure you're getting consistent feed. The two biggest factors I found with MIG on ally was the angle of the feed (15 deg from perpendicular) and I found a brand of wire that really worked better than what I started with. The cleanliness of the wire may have had a lot to do with it also. There's some good tips here
http://www.millerwelds.com/resources...cess-at-Home-/
Sounds like you might want it a bit cleaner and hotter to start with then travel a bit slower for more deposition and cooling.