The metal has to be super clean! You might try using .023 wire, have your stick out on your gun a little longer (about 1/4") and touch the side of the wire to the inside of the hole, pull the trigger. Let that weld spot cool and do the same on the opposite side and go back & forth until the whole thing is filled. I just welded shut 14 trim holes in a door w/o any issues, and didn't need a backing plate...BUT I am using a welder that has finer tuning than yours and solid core wire, & gas...I gave my Hobart Flux Core welder to a friend and upgraded after doing the same thing you're having issues with.
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Janine
1965 Chevy Step Side Short Bed
