Sheetmetal welding is tough. The thin pieces are easy to burn holes through, and it’s very difficult to fill in any edge gaps that didn’t come out well. A copper backing plate is a very useful tool to have in your welding supplies. It dissipates the heat so you don’t burn through the steel. It also acts as a backing plate for the weld bead to pool up on nicely. The best part is that the molten steel won’t stick to the copper. Copper backing tools can be purchased at your local welding shop, or fashioned from pieces of copper pipe crushed flat in a vise or with a hammer.