If I could add to what Shiers said - don't leave a gap between 2 pieces of thin sheet metal, it only makes blowthrough more likely. Body sheet metal is so thin that if you make the seam tight, the wire will still penetrate and make blowthrough less likely. I've found I like a little more heat than factory settings on the same wire speed. For me, it helps the weld to penetrate and lay flat. If I used factory settings, the weld would always be raised and not penetrate as much. You do have to move a little quicker though. It's all trial and error to see what works best for you.
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