MIG and TIG welding works best on clean metal. Stick welding and Acetyline burns the paint away ahead of the weld, so it's not quite as critical to get picky about cleaning. It sort of depends on how smooth you want the finished product to be, and/or how much time you're willing to spend grinding and polishing the bead after the weld is done.
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!