Well, it's not like they have a magic wand when they or someone else washes them either to prevent that from happening. It is likely that this potential customer already accepts that this will happen. It's definitely worth some discussion with them at the very least. If I were you I would try and throw a bid out there of $15 or so if they clean them at least once a month. The more they have them washed the less of a dirt build-up under those rails you will have to fight. Hey, who knows? Maybe you can talk them into going every two weeks, but I doubt it with the way that schools are tightening their belts...