Ooh, I really like the idea of soaking the parchment in various potions! Or perhaps it could be added to the ink? I hadn't thought of that sort of thing. Thanks!
I also quite like the idea of the Impervius and the stealth one. Impervius is something I can see them adding after the map has been finished as one of them realises it could get wet/it does get wet or leaked over.
Thank you very much for your ideas! If anyone else has any, it'd be great to hear them.