mig welding sheet metal

Im replacing quarter panels on my car and have been practicing butt welding on the old panels. I have been having problems with burning holes through the metal, I have tried to weld for a second then stop then weld It works to a point and sometimes I get a nice weld but usually I burn a hole in the metal, also I been stitch welding the metal and it is still warping in some places. Any help would be appreciated.

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a few tips...
small 115volt mig welders work best for this
use .023" wire
use a little less wire speed than usual