Back the crack with a flattened copper pipe and you can fill in the gap without the weld sitcking to the copper. Then grind the weld down. I use a folded up and flattened piece of roof flashing to back butt welds on the body repairs. Great help on metal with thin spots to keep from burning a hole through. It sticks sometime but can be pulled loose by hand. The flashing flexes enough to conform to the shape of the repair when I clamp it down. Smart flix has some great videos on how to adjust the welder. I Think the rental is $9.99 for a week. You tube also has some videos on welding and body work for free.