I used a higher speed (.023 wire with shielding gas) on my sheet metal patches which were a bit thinner than 16ga. I went to Lowes and got a 2' x 2' sheet of steel and cut it up into 4" squares and went to town. If you're going to be welding up panels, try to get a gauge similar to that which you'll actually be butting together. Here's what I managed to do after a little practice:
http://picasaweb.google.com/ChrisAurit/PhotoHistory#5377946874362322594http://picasaweb.google.com/ChrisAurit/PhotoHistory#5377947232745876514http://picasaweb.google.com/ChrisAurit/BeforeAndAfter#5224683405793990546These were my second or third attempt at butt welding. I think I did the floor panels first because the visibility factor left me a lot of leeway.
I also used those butt-weld doo-dads that Eastwood (and others) sell that really helped me line the panels up.
