Think I know what you mean. Its just like baked on dirt and brake dust. Have it on mine but I'm spraying them up at the moment, and first step was to scub with hot soapy water using a scouring pad which has removed them. Obviously dont do that as you will remove the top coat, but you may need some sort of cleaning compound with slight abrasives in if nothing else seems to work. Sure someone can recommend something along those lines as I dont have a clue sorry!