Selected Answers

From Ted Valoria of Sponge-Jet, Inc. on November 15, 2016:
Sponge blasting is an ideal solution which has been used to remove graffiti from sandstone, granite, concrete and brick successfully for many years.

From Jesse Melton of Harpers Ferry Toolworks on November 14, 2016:
The question is kind of vague, isn't it? Historic ...read more The question is kind of vague, isn't it? Historic implies solutions that don't create problems for future work. If that's what people are really after, the best answer is exposure to a delocalized fusion emitter and salt baths comprised of locally produced airborne particulates. This allows the appearance of the stonework to be consistent across the graffitied faces while preventing harsh chemicals from being introduced into the stone and leeching out to create a stain that never goes away. If short-term is what people are looking for, erect a media blasting cover big enough to cover all the stonework on the graffitied face(s) of the building and do the blasting during a consumer-focused holiday (Thanksgiving and Christmas, for example). Leave the tent in place for 6-9 months after the blasting and Bob's your uncle. Humans have great difficulty accurately remembering color (that's why people still paint stuff red), so they'll never notice the change once the tent is down.