I always use a pressure washer on the black plastic bits. It is very important not to get polish on the plastic or it will discolour it. If you put the plastic/vinyl care product on first it will help to stop the polish contaminating the trim should you accidently catch it with the polish.
I have seen a lot of MINIs with spoilt black plastic trims caused by over liberal application of polish. Why they had to give the plastic a textured surface I do not know! |