Actually, an inexpensive way to do this is to mix 9 parts water to 1 part feed grade molasses. Handy when you have big parts to do. Something in the molasses makes the rust water soluble. It might take a few days or a few weeks to remove everything, depending on how rusty the part is to begin with. You will need to coat the part with something when you take it out of the solution as it will surface rust pretty quickly.