I have a Lincoln mig welder that I just converted from internal flux,.035" wire feed to .023", argon/CO2, mig service. I am welding a lot of sheet metal pannels together with it and having surprising success. Penetration is great but the weld is extremely hard as mentioned by other posts. As mentioned in one of the posts above, multiple spot welds are much better than trying to run a bead of any length. You can gauge if you are going to have panel warpage by the blueing of the metal aroud the weld. By spot welding 1" apart, then 1/2" apart, then 1/4" apart and so on, the blue area on either side of the weld bead is almost non-existant, thus I have achieved zero warpage in large relatively flat panels. However, if I try to run a bead of any lenght, the blue zone goes to about 1/4" to 3/8" and I end up with warps. They are not as bad as with a gas welder but still there. Since I am the worst metal finisher in the history of the world, avoiding warping in the first place is my only hope.