Don't try this but one time I took my trusty "White Pearl" eraser to a proof silver coin to try it on the water marks. It worked. I only did it once, and it stayed nice for years but I finally sold it.
I'm guessing that neither you nor the buyer looked at it under light from the right angle after the eraser treatment. It's hard for me to imagine rubbing a proof coin's mirrored surface with an eraser and not leaving hairlines -- even an eraser that doesn't have an abrasive as part of its formulation.
If you do anything please report the results. I am guessing that the vast majority of collectors have similar issues.
@jeffB Funny you should mention that, I did look at it under a Dinolight & my loop at many angles and it looked fine. I wish I still had the images but I did delete them. Like I said don't try it at home.
throw it in the dishwasher? How do they clean the coins in the mint. I doubt they just stamp them and send them out without a wash.
. The guy at a jewelry store down the street from me cleans coins with the same steam blaster he uses for cleaning diamond rings.
If I had to guess, I'd say that someone sneezed or coughed on it between the time it was struck and the time it was wrapped.
Clearly though, the first step is an acetone bath. In fact, that might benifit most of my coins. (that is a question not an answer). I can't really hurt them even leaving them in a bath fo hours.
I think I'd try distilled water before trying acetone, but acetone first won't hurt. You're right that it's harmless. (Don't leave them in bright light while they're soaking, though; there's a remote possibility of light-catalyzed reactions that can change the color of copper.) If the spot's been there for a long time, though, it's likely that some toning has happened, either under the spot or everywhere-but-under the spot. That's harder to fix, and requires a chemical approach.
. It's steam. What could it do? I've since done it lots of times with coins for the "Coin Club"s at the schools I work. Now of course the coins I'm cleaning were never ever gonna be worth anything more than face value anyway unless the get scarce for some odd reason. They don't show for anything done to them except for looking clean of any garbage that may have been on them. cplradar "that doesn't damage them?"
It's steam under high pressure, right? What if there's abrasive material on the coin, and it gets blown across the coin's surface under pressure?