.023 solid wire gas shielding is the way to go for sheet metal, much more control and cleaner, my 2 cents. Also flux core and solid cord use diff polarity should say on or in your welder.
You can use a little thicker like .030" on floor pans, and frame repairs, but on exterior sheetmetal, I have found .025" solid core w a mig welder works much better than the thicker wire.
