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