Here's a bit of a before and after Mantle scenario for your perusal.
I bought the 1968 card on the left, because the stain was simply gum or wax residue (in front of his nose and mouth) that I knew could be easily removed with pantyhose if the occasion ever presented itself, so it was worth the price. Fast forward to Bobby's group sub, and I cracked it out and wiped away the schmutz...and it (card on the right) came back a straight PSA 7...
1968mantle280psa7beforeandafter.jpg