After a quick Google I learned that the appropriate method for MIG welding for sheet metal was:

1. Use 0.30 solid core wire with Argon/CO2 gas.
2. Space welds about 1" apart allowing piece to cool to prevent warping.
3. 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.