After a few weeks in the booth, i took a 51 Merc headlight ring in to practice on pot metal. That metal doesn't change colors. It just goes away with no warning. After a couple days of practice, i invited one of the instructors into the booth to try his hand at it. He had never welded it before and even though he was a good welder, after ten minutes he threw up his hands and walked out. That was funny. i learned later that a different rod and technique work a lot better....
TIG weld it just like you would aluminum except that you will only weld for about 5 or 10 seconds at a time before stopping to let it cool. Just when it gets to flowing good. STOP! Its about to melt and fall in your lap.
Recommended rods are:
Alladin 3 in 1