Not a permanent solution, but I find the coloured wax polishes work well. For my Sterling Grey Z4 I mixed silver and black until it was slightly darker than the paint. I then apply a coat to the troublesome areas (front and back corners of sills and front bumper) and let it dry, then go over it again with another generous coat. Allow to dry and then buff the lot off. Works wonders, and with careful washing with a decent shampoo lasts a couple of months.