I used my mig for the same aluminum welding, using aluminum wire. This was my first time welding aluminum but I had gotten some advice from this forum so I figured I had someone to blame it on if it all went wrong......
I found out you really need to move much faster than you think you do and your temp has to be adjusted just about perfectly. I had to stop about every 3-4 inches of weld and clean the aluminum again and again to keep a clean weld surface or it just wouldn't lay down a decent bead. Also, your joint has to be very tight or you'll burn through at the gaps every time. Once I got the hang of it, though, it went pretty well and I saved myself having to buy a new sheet of . 080 aluminum.