What size welding rod are you using? The smaller the better, IMO. If you have say a 1/8" to 3/16" hole on 20 gauge carbon sheet metal, you should use a block of brass or copper clamped to the work behind the hole(s) and all you'll need to do is "plug it up". If you don't have copper or brass, you can still plug up the hole with just the torch and wire but you have to finess it in. Use 1/16" rod if you can and set your heat hot enough to melt the base metal, but just barely and keep working back from one edge of the hole to the other until the hole gets smaller and smaller until it's sealed up.