Ok, to follow are my attempts to conserve milk spots on this coin. methods: liberal wash with mild detergent and copious rinse ammonia “dip” and rinse vinegar “dip” and rinse jewel luster short and then long dips with copious rinse electrolytic reduction with hot vinegar, aluminum foil and baking soda electrolytic reduction with distilled water, aluminum foil and baking soda notum: the above reduction chemistry is well documented in literature and essentially the “reverse” of oxidation I then returned to the jewel luster dips with copious rinses here is what was left: picture
Im thinking carbon spots which wont wash off they may change color but will return . There's no cure...! Other than sell the coin and find another. They can surface at anytime even after a coin is slabed. The Jefferson nickel was found in a roll of 40 smack dab in the middle, and had no effect to the 2 coins next to it. When medals leach out carbon or other alloys or components there isnt any fix.
Yikes. I really do believe that milk spots, though not as bad as your poor oxidized nickel have no true remedy. I really exhausted ALL of my resources, and only gave a sketch above of what I did. At best there was only VERY slight improvement.
I assure you you cant fix it...they never disappear they may change color , or lighten...but there's no cure. I had an 1934 D buffalo nickel ms 67 slabbed by pcgs 6 months later spots begain to surface. A $600 coin now a make me an offer on it..... Im sorry to tell you... as I know the pain.
You did all of that on that poor coin!!!? For milk spots, the only things I would ever trust or do would be acetone/xylene soak/dip and caustic (NaOH or KOH) rinse/dip.
Actual milk sports dont have any fix, and if they do who ever knows how to do it isnt saying a word to anyone. Theres a ton of money someone could make cleaning them up quietly after buying them at a discount. PCGS even has/had a reward if someone came up with a fix