On my terms, only "natural" substances i will remove on a normal clean like bird muck, tree sap etc, otherwise i will charge a half hourly rate (depending on how long it takes) for anything that needs to be removed that been put on them by "someone" else, like paint, stickers, waterproofing etc.
Its an additional service in my eyes.
edit..
those non-natural substances should be removed by whoever put them on there in the first place.