At some point sooner or later your car wash will be a target for graffiti. A few ideas for cleaning graffiti from a car wash that work quite well.
Paint stripper – Be sure to get a good quality so you don’t have to do the job twice, but the paint stripper should weaken the graffiti it and allow you to remove it with normal bay pressure. One product in particular is from Grainger called Amnesty. It is an aerosol product and works great on things like signs, walls, stainless, etc. Good Luck!
Another source of graffiti removal could be through the city or fire department. They can get ahold of special stuff and sometimes give it free of charge because the graffiti makes the entire city look bad.