I used CO2 ( pub style bottles ) to weld my chassis,
BUT
After recently getting a bottle from Vmax of Argon Mix & connecting it up to my Mig & having a play/practice welds, I wish I,d used the Argon Mix ,
I,m new to welding ( went to Night School to learn the basics to start my Roadster Build) & just seem to find it so much smoother ( not sure how to describe the difference) when using the Argon Mix, the weld pool seemed to flow so much easier, hardly any splatter at all, come to think of it none at all.
Hindsight is a wonderful thing eh

( It was suggested to me at the beginning of my build to use the Mig mix but I found it nigh on impossible to obtain without getting involved with a rental contract etc

, so CO2 it was to be )
I now have a 5ft Bottle that will last me ages & ages
cheers
andy