Just found a great product while shopping at the second-hand shop. It's paint on rust! It's applied in two parts, first part has actual iron dust in it and part two causes it to oxidize.
I tried a couple of quick samples just to see how it is going to work. When I get the hang of it with a little practice I think this is going to be a pretty cool technique.
I don't know it this stuff is currently available but it might be worth checking into.