That postal ink all over the postcards...
If you mean the flouro orange/pink stuff theat Canada Post uses for its automated sorting / routing information..
This may gross you out, but I lick it off, and then lick my tongue on a kleenex to capture most of the ink that has been removed.
The tongue has the right roughness, and is just the right amount of wetness, that I have not been able to relplcate with a conventional moist sponge or rag.
The marks come right off, and the emulsion barely swells at all. Usually there is no sign of that routing mark was ever there within 30 seconds of this unconventional cleaning treatment..