I had some extremely rusty/nasty parts to deal with. I used a 10:1 mix of water to molasses and let the stuff soak for a few weeks. They came out very nicely, and it cost me less than a dollar.
I've never heard molasses before, I'm picking up a bunch if vinegar on my way home today and a tub for these and am going to let them soak while I wait to see if my uncle finds someone who welds cast before I try it myself.