I used some "Bartender's Friend" Oxalic acid based paste on a rusty spindle once the rust went away, but it actually turned most of it a dark bluish black, very similar to *** bluing. I was careful not to use too much and dilute it with water and not to leave it on too long, but the spindle and actually, the cups turned blue/black on me. Looks like it's just cosmetic, but wondeirng if anyone had the same experience....
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