The thinner wire the better, and "stitch weld"- - - - -make short passes a couple of inches apart to avoid distorting the panel with heat. Let things cool down, and split the difference between welds the next time around. Eventually you'll get a solid bead with a minimum of heat distortion. Gas shielded solid wire will give a MUCH better weld than flux core.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!