After a quick Google I learned that the appropriate method for MIG welding for sheet metal was:
- Use 0.30 solid core wire with Argon/CO2 gas.
- Space welds about 1" apart allowing piece to cool to prevent warping.
- Once the welding is finished you can grind down the welds, go slow to prevent heat build up and warping.
Below are a before and after of a crack I welded up on my project.