Spending time on a rusty chain doesn't make a whole lot of sense to me because they cost so little. I used to do a lot of work on lightweghts and bought chain from a bike shop by the foot. Much cheaper that way so I ususally got enough to do 3 or 4 bikes and always had some on hand. Nice to work with a clean chain and it only took a few minutes to install a nice new one..
However if you are working on a skip tooth or want to keep the bike totally original that's a bit of a differnt story
I had a skip tooth once that was a bit rusty but not pitted or damaged the point of unsafe. Got all the grease and crud of with Easy Off oven cleaner. Just sprayed it, let it sit for a few minutes, then hosed it off. Used a heat gun to dry it rhoughly and quickly.
Next put in in a pot with an oxalic acid solution and let it sit for about an hour.
It came out looking looking like bare metal (which it was) so I put it in old pot with gun bluing solution. Warmed the mixture up using my trusty heat gun and let if sit for about 10 minutes. Pulled it out, rised with water as per directions, dried it throughly, then oiled. Ended up looking pretty darn good and the whole process only took about an hour of actual working time.