They change them out to fast here to clean - a very old sign would be a year old.
about 15 years ago, I did a job for Viacom (now CBS outdoor) to cut out rusted cat walks, replace with new, grind and paint the pole (60 ft about 10 ft around) and paint the T- 111 at the base (4' by 48 x 2 sides). It was a painful learning experience, I thought I was going to make a fortune but instead lost my shirt.
I would say you would have a better chance of doing that type of maintenance rather than cleaning the sign