Welding wire ?

Adam, as mentioned, use the .023 wire for auto sheetmetal with 15 to 20 #'s gas pressure. Also, make sure you change the tip on your welder to a .024 so the wire isn't flopping around in the tip, and tighten the tension up on the feed motor to compensate for the smaller wire. The biggest trick to MIG welding on a car is SHORT welds. Pull the trigger, say 1001, and release the trigger. Also remember that a MIG is a cold start arc, so if you're welding two different thickness of sheetmetal together, start your weld on the thinnest piece first. Keeping the ground clamp as close to your weld area as practical also helps with these small units. Practice, practice, practice! :)