Quote Originally Posted by Trip59 View Post
Don't be afraid of a little hammer/dolly work as you go.
this is essential in sheetmetal welding. the weld shrinks as it cools, you need to hammer it back out. best way is to grind the weld down to just a bit proud of the parent metal, then hammer. you should do this after each pass when you're skip welding (which you should be if it's 18ga or thinner). of particular note when skip welding is the ends of your welds. the ends will pull metal along your weld seam as well as perpendicular to it, making a pucker, so you have to work them a bit more to get it flat.