I noticed this evening that the numberplate on the BMW is filthy. It's covered with greasy muck and is almost unreadable. What's the best method for getting it spotlessly clean? I was thinking of trying a jetwash to get the worst of it off, followed by hot, soapy water and a sponge, but I don't know if this will cause any damage to the surface. Also, is there any sort of lacquer or treatment (Rain-X?) that will help to prevent this build-up of dirt in future?