you need to have both pieces grounded to weld good. also you shouldn't have much if any gap! wire wheel the old steel and use weld through primer. also your burning through because your heat is set too high! use more wire. put the welder on heat setting #2 and the wire on #3. you should have more wire than heat when it comes to sheet metal! make your tack weld 6" apart and go around the whole panel. do this till its warm or you've completely welded the whole panel. don't get the panel too hot or it WILL warp!