I have had alloy wheels previously, but on my new Mercedes C200K they are very difficult to clean, the worst feature being that the "dust" has hardened on in small specks, and seems impossible to remove. I have tried all the usual methods: polish, white spirit, meths, but they have no effect. However I did see a spray in Halfords which claimed to do the job, but it seemed to have a lot of warnings attached. In fact it appeared to be some sort of acid. I should appreciate any comments.