Evaporate is good but expensive. Get some phosphoric acid, E Bay etc. You want to immerse it so find a suitable container, spaghetti jar maybe for something that shape, or a short length of drainpipe capped at one end. Degrease thoroughly first and pop it in the jar then fill up with acid. Check after 15 minutes how it is doing, then you can assess how long it needs to stay in. Once the rust has been removed wash it in hot water and a bit of washing up liquid to remove any acid, wipe it down with some kitchen towel and immediately give it a good spray with WD40. This will drive off any remaining water and prevent it immediately rusting again, which it will If you can't get it dry immediately by using an airline or similar. Then you can give it a going over with wire wool, wet and dry, or whatever to get a nice finish. I usually use a wire wheel on a grinder, lubricate the part with some light oil, diesel is good. Then finally clean it thoroughly with thinners to get rid of the WD40 and residue from rubbing it down. If your workshop is dry that's it. If you have problems with damp then coat it in wax polish, or whatever product you favour for protecting it. Don't use WD40 for this as it dries out over time leaving a nasty residue. The advantage of phosphoric acid is that it dissolves rust very rapidly, but only acts very slowly on sound steel. It will remove every trace of rust without damaging the part, and it's cheap and easy. Make sure you wear rubber gloves and goggles. You typically buy it concentrated, and can then dilute it for use. It is, or certainly was, the active ingredient in Jenolite mentioned earlier. As usual nowadays most of these products have had the active ingredient reduced for health and safety reasons to the extent that they take forever to work.
