Long time no see, CT! Decided to go to the LCS yesterday, something I can’t do very often right now with lots of essays/projects due in place of exams. Spent about $30 on junk silver dimes/quarters, added a bit more to my PMs stack ($1.45 FV in total). The Mercury Dime tray was on top of the case, implying someone had just looked through it as I entered. Decided to see if they had missed anything. After a bit of searching, I came across one with AU level details (some mint luster on the reverse too) and very dark obverse toning - in the low-light coin shop, I could see the S mintmark and that the date was 192-something (definitely not a common 40s Merc!), but looking through a loupe showed the last digit was a 5. Unfortunately, there is a scratch on the obverse in front of Liberty’s nose, but with these details, still worth quite a bit more than 21x face. Didn’t find another 34 DDO in the quarters, but I found this. When you’re looking through lots of circulated 30s and 40s Washingtons they just stick out like a sore thumb when you find one.
Acetone probably won't touch it, but some heavily diluted dip might lighten it. I don't think I would try removing it completely. I think it would have a "dead white" appearance if you did.