What's the deal on tig welding radiators, Had some an fittings welded on and the welder said he had problems. The radiator is used nascar one, beautifully tig welded, now not so much.

Clean, clean, and clean some more. just like anything that is aluminum and needs to be welded. There are preweld cleaners made just for aluminum that clean very well that remove oils and oxidation. I am sure the antifreeze is a pain to get completely cleaned off, but it has to be done. The only radiator I have welded was a new griffin that was epoxied together at the cores and I had no problems with it. Just welded a side cover for a quad today that was very oily before cleaning and it came out great. Washed with alcohol and then the preweld cleaner. Beep

What he said..Likely the cause was contamenents from inside the radiator. Not much you can do in that case. best to start with a NEW radiator if you are gonne be doing any welding on it.

Clean it GOOD with a stainless brush and clean the
color off the fitting
EDIT
I do the welding first then drill the hole so you
dont get trash coming out of the hole

i did radiators for a living for 1/5 of a century. once aluminum has been in service, it is corrupted, and requires different methods. beyond a certain point, they are no longer weldable. like alguire said, the problem is from within, mainly.