I remember they cleaned the paintings lining the Sistine Chapel some decades ago. Big, big controversy at the time, people were afraid the art would be ruined. They said at the time that the cleaning came out great.
I remember thinking back then that they should have waited. As time goes on, more devices, electronic and otherwise, get invented and used, and I figured if they held off another 50 years, they would have a much better chance of cleaning the paintings with no harm. After all, the paintings were hundreds of years old, what's 50 more years? But they went ahead and apparently lucked out, it came out all right.