There are several rust removal methods out there. I've used Gunzilla by placing a rusted part in a small container of it, and over about a month, and all the rust falls to the bottom of the container. Much faster are
Evaporust and
Rust-Release, both of which use a chelating agent to neutralize rust by conversion. Rust Release is slightly more acidic, so it is faster, but it also can leave the surface a little more able to re-rust promptly if you don't do something about it. I recommend a soak in mineral spirits or naphtha overnight to degrease before using either product and then let the solvent dry completely before putting it into the compound.
Acids will all take rust out, too. For example, oxalic acid is an ingredient in some ultrasonic cleaning solutions for deoxidizing. So is Citric acid. Phosphoric acid generally leaves a very thin protective layer of phosphate conversion on the surface, so you have a couple of days to get to whatever finishing you intend to do. Brownells Oxpho-blue and Van's Gun Blue are phosphoric acid-based cold blues, and if you want your dies black, you can use them to neutralize light surface rust. Neither makes as dark a blue as the nitric acid-based cold blues, but they are the only ones that produce no after-rust if you leave the un-oiled blued item out.
With any of the above treatments, I have found it helpful to apply them with an ultrasonic cleaner. First, you put the solution in and then the part and run a degas cycle. That tends to free air trapped in the rust and let the chemical get at it. Then run the ultrasonic in normal mode for a bit to knock any loose rust or oxide off the part.
With any of the acids, I want to neutralize them as fast as possible when the job is done, so I spray the part down with Formula 409, which is quite alkaline (don't apply it to aluminum for that reason, or it will etch it) or a mix of baking soda with a little dishwashing liquid. A teaspoon of each in a quart of water is enough for the remaining traces of acid. Then the part gets a very thorough rinse in hot tap water, and then I boil it for ten minutes in distilled water. That does three things: Trace rust formed on the surface by contact of the bare metal with air is converted to magnetite (blue iron oxide). The minerals in the tap water are dissolved to prevent water spots. The hot part dries rapidly upon removal from the water, and any new oxidation that tries to form during drying also becomes a blue or yellow oxide which you can see if you angle the part in the light correctly. You can then submerge it in a water-displacing oil to get trace water out of any nooks and crannies, or you can set it in an oven a 250°F long enough to drive any water out. That temperature will not adversely affect steel temper.
We shouldn't leave out mechanical removal. A stainless brush wheel, even one on a Dremel tool, can be loaded with white buffing compound and run around the threads to clear rust and polish the threads, which helps prevent future rust. Follow up with a clean wheel and Flitz to get a final polish and a little wax left on the surface for further protection. Even if you remove the rust chemically, polishing and waxing or treating it with a rust inhibitor afterward can help prevent future rust.
