The problem is that most chemical, HCL, soap , acid and other things can destroy the water based paint they use on these buildings.
Scott, believe me the drivit can handle some pressure. The sign will be ok also, we kind of talked about this at the round. The pressure thats actually at the end of a wand is way less 8 inches from the end.
Try something simple like a mild soap dwelling first. FOAM with out caustic.
going after this with something hot will just embed it into the paint further.
Several light treatments....