I don't know if you've heard of an ancient piece of rock art in Utah called The Harvest Scene in Nine Mile Canyon, but if you have, you've probably heard that there's been some difficulty lately in preserving this panel particularly but the whole area generally. The problem specifically is that the 800 year old artwork is being covered with dust stirred up from the commercial oil/gas trucks driving through the area.
The local county's solution to the concern about the dust? Yes, you guessed. They sent a crew out to wash the panel. With soap and water, you guess? No. Try guessing "a pressure washer".
I wonder if they'd have been allowed to wash The Last Supper with a pressure washer because it was starting to look a little soiled? Heck, it's a newer piece of art by at least 300 years; it can stand it.