Let me see if I can make GMC248's excellent response more confusing!
Remember that increasing the wire speed increases the amps, which results in more heat. Increasing the voltage setting, (1,2,3,4, or A,B,C,D) also increases the heat. Conversely, reducing either dial decreases the heat. Moving one adjustment usually requires adjusting the other, too.
When I'm trying to help someone learn to weld, the most glaring thing I find is that someone learning doesn't put in enough tack welds prior to welding the seam, which '248 mentions. Consequently, the 2 edges of the seam are allowed to distort during welding. I recommend tacks about 3/4 inch apart before actually welding a seam. Make sure that the 2 edges are perfectly flush. Any discrepencies in the seam are going to make it difficult to make clean welds. Then, it's easy to stitch the panel together intermittently like described.
MIG weld will tolerate a little gap in your seam, but it should be pretty close... no more than the diameter of your wire.
Unlike '248, I like a 110v machine for sheetmetal work, and I run .023" wire. I find for my welding, that a larger wire requires more heat, meaning potentially more burn throughs, or, a "prouder" weld that requires more grinding. Even if your machine is 220, you should be able to dial it down. Try lighter wire, if you're not already using it.
GMC248 gave the best advice.... practice, practice, practice. Taking care at the front of the job to fit you patch carefully and accurately results in a faster, nicer installation that requires less clean up as an end result.
Tim
