Can't help you with anything about Fabshield 23, but you should be able to patch sheet metal just fine with a good MIG welder. I never use flux coated wire though - I use solid wire with Argon/CO2 shielding gas. Just a matter of turning down the amps and the wire feed to match the gauge of the metal and it works fine. For sheet metal patching you use a bunch of full penetration tacks, spacing them out to minimize warping, same as TIG - tack, move down an inch or two and tack again, continuing until you've filled it in with solid weld all around. TIG's probably easier to control the heat, but MIG will work once you have the settings right.