I had a nearly identical situation. Bought a PSA 7 ST '68 Mantle, and a year or two later cracked it out and easily wiped away the gunk (yes, with panty hose) and resubmitted it. It came back nicely as a straight 7.
Check out post #1441 in this thread to see before and after scans:
https://www.net54baseball.com/showth...222292&page=19