Great question — answering as the UphillTech team:
Liquids might damage it. The following suggestions are from mildest to more likely to damage the hat: Various types of erasers . The most gentle I've used is a bag of shredded gum rubber , AKA "bunny bag". One blots, as much as rubs, with that tool. Exposure to sunlight (or UV lamp). This is less predictable in removing color. Gently spraying with with water and then exposing the hat to chlorine gas. Unless one is familiar with safety precautions for handling toxic chemicals, leave this to a professional. Spraying bleach (e.g. sodium hypochlorite) directly on the hat might remove the stain, but the dried residue could cause the paper to disintegrate later.
Full answer by DrMoishe Pippik: https://lifehacks.stackexchange.com/a/15682 (CC BY-SA 3.0).
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