One sneaky trick to prevent burn-through when butt welding sheet metal patches is to clamp a piece of sheet copper behind the butt joint. I use a piece of large-diameter copper pipe, split and flattened, and held into place behind the butt joint with c-clamp vise grips. The copper wicks the heat away quickly, which allows longer passes to be made while stitch-welding without distortion or burn-through.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!