You can use a flux core welder on regular sheet metal. If the metal is old or thin, you can use a copper spoon held behind the weld or seam to wick away excess heat and prevent burn through. I did about 50 holes on my 47 jeep this way. If you follow this link:
http://www.stovebolt.com/gallery/alternate/arnstein_irwin_1947jeep.html you'll see a picture of a spoon I made with a hunk of copper and an old screw driver that works just fine. If you have a small hole, you can just use the spoon and spiral in the wire as the weld wire won't stick to the copper.
