The mention of boric acid prompted a vague memory.
I gave a substance that has been used in some rust removers. Oxalic acid. 15min. A tarnished item with slight rust patches near the end that was immersed. It converts rust to a water soluble oxide – especially when the water is hot. Sadly I didn't take a photo first.

The immersion line can be seen up towards the slot for knocking out a morse taper. It's a 1 to 2 jump up sleeve, came in the drawer with my miller. No idea why some one would want one.
I then swilled in hot water and used a kitchen type scotch pad on all of it including the morse taper with hot soapy water.. I suspect the taper shows why some people use wet and dry plus paraffin. Parts are oiled and some always stays there even under rust. There is still some pitting which can't be seen in the shot.
The results look much better than the usual rust remover that can be bought which leaves a very grey surface which is fine if it's to be painted but not much good for machine dials. The usual stuff sold is cement cleaner – dilute hydrochloric acid.
If any one wants to try it I used a saturated solution and it would be best to read the wiki on it. It's odd stuff often in things we eat and is poisonous to a degree and can be absorbed through the skin. I use it as part of a process for oxidising diatoms for viewing under a microscope. With a couple of other chemicals it's a lot safer than literally using boiling sulphuric and nitric acid.
Might be good in an ultrasonic bath but I vaguely remember causing some upset by poring it down the sink – it may stain stainless so would have to be used in glass in a bath.
John
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Edited By John W1 on 17/06/2015 23:26:43
Edited By John W1 on 17/06/2015 23:27:31
Edited By John W1 on 17/06/2015 23:28:16