General Cold Front
Welding thin panels, especially at the edge (window frames come to mind) it's very difficult to prevent buckling
Even with a good MIG on low setting, it's a good idea to stop after each spot weld, using a damp cloth to quickly cool the area before continuing..
You can get a cold putty from Frosts called Cold Front that sucks heat away locally, put it around edge near weld to stop distortion, to some extent can be re-used. Use an 80/20 or 95/5 Argon mix* on low power I'd think and keep stopping, do it in short bursts.
I think Cold Front works by evaporation, its like a menthol putty :-)
- CO2 seems to need a lot more power!