When butt welding on sheetmetal just tac and keep moving so You don't get to much heat on the work say 6inchs apart than go back and tac between those welds and so on. To much heat in one spot will warp the sheetmetal don't try and weld it all up at one time better to let the work cool down than tac some more also a little more speed on the wire will give You a colder weld but there is no substitute for practice I got some junk sheetmetal from some old cars and cut it up and practiced alot .
Pete
