In my experience that'd be electrolysis or Evapo-Rust.
Buying enough ER to completely submerge the part is a bit expensive, so especially if you already have a DC power supply such as a car battery charger, then electrolysis will be the cheaper way to go.
There's a gel form of ER that clings in place, but it's not as effective as a complete dunk in liquid ER. Heating the solution also helps, speeds it up anyway, and if you can pull your parts out of a hot dunk then they tend to self-dry, so you don't get flash rust from it coming out wet. Obviously dry the parts ASAP, such as by blowing off with compressed air, warming with a heat gun or torch, spraying with WD-40 or however you you normally dry your ferrous parts.
Both ER and electrolysis assume you have disassembled the mill. If you want methods for doing it in-situ, sorry I got nothin'. Scraping with a razor and/or rubbing with steel wool soaked in WD-40 are common methods, but there's more chance of harm, and you can't get the rust in all the crannies.
I am not an expert in vintage machine restoration so I'm eager to hear other people's methods.
