I would like to repaint just the fin half of this radiator that is rusted. I would like to know of there is a special treatment due to rust that I should do. I do plan to sand the rust with a grinder and a flapdisk.
that looks like just surface rust. wipe it with strong citric acid or some commercial rust remover (like CLR or Limeaway). if it all comes off, its just some rust that has dripped down from the upper port. assuming there are no current leaks, you can just wirebrush it off and seal with a rust converter (corroseal or 3m marhyde are by far the best at this job) and then repaint the whole thing with a good quality latex or polyurethane (it will flex when heated).
just be gentle. remember, a radiator is a pressure vessel full of hot water.