Originally Posted by
gmwillys
The key to welding sheet metal is to make a series of spot welds. Start at one side, weld a spot a couple inches apart, then go to the other side and repeat. This keeps from warping the steel, and causing you headaches. When you have all your welds two inches apart, then you can spot weld in between, following the same strategy, until the weld is connected all the way around. It takes a while to complete, but it will turn out well.
Are you using flux core wire in your mig? If so, it takes a bit more heat to lay down a good weld. When you get the copper, you can back the weld surface to help disipate the heat, and to help bridge the gap.