Seal the seat tube and the head tube were the post and stem are inserted. Turn the frame upside down and fill the head tube and the seat tube with amonia. Best to do this outside because of the fumes. The ammonia will break down the steel/ aluminum oxide bond between the stem/post and the frame tubes. Let it soak for a couple of days. You are more than likely not dealing with a traditinal rust situation so petrolium based lubricants will not do very much.