Q.
When welding butt welds on floor pans, what size MIG wire should I use, and at what voltage setting on the welder?
A.
When MIG welding automotive sheet metal, thin wire is better. Most often .023 solid wire is used with a 25/75 (argon/CO2) shielding gas. Some welders can not use .023 wire, so .030 wire can also be used instead but watch for burn through. The voltage setting is different on every welder and welding condition, just make certain you are getting good weld penetration without burning through. 
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