We have noticed a yellowish film forming on our rear bumper and tail light cowlings of our 2011 Flying Cloud. The bumper appears to be chromed steel and the cowlings appear to be chromed aluminum. The yellowish film can be scrapped off using one's finger nail, but it is not easy to remove otherwise. I,ve tried chrome cleaners, a plastic scrapper and cleaner wax. None of those put a dent in it. I don't think there is a clear coat on these parts, and if there is a clear coat, i don't think it's old enough to be deteriorating into a yellowish film. So, what is it, and how do i get rid of it ?