I would agree with Cory. I've soaked cards in distilled water and very gently wiped crud off them with a q-tip. Even those steps, some folks treat as "altering." I kinda shrug at that purist attitude, because I like my 115-year-old cards to look their age.
Anything that uses chemicals may show up as part of the grading process and result in either an "Authentic" grade or the card being punted as altered.
Writing won't usually come off well, and it's worth noting that there is a community of collectors (like me) who actively pursue t206 cards with period stamps on them, so anything that might remove one of those would remove some collectibility.
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