You don't use any kind of gas with flux core wire, it is self shielding. Flux Core has a flux in the center of the wire. The wire is actually hollow.
I use .023 (.024,.025) mig wire for most sheet metal stuff, .030 for heavier stuff. Some on here use .030 mig wire for welding in patches on these trucks since the metal is 18-19 gauge stuff.
Flux core isn't very good for welding sheet metal, leaves a dirty looking weld, similar to what you might get with stick welding.
Have you checked to see if there is a kit for upgrading your welder for shielding gas? What brand welder do you have??
John
