I have some black and brown spots on the GRP bodywork of my MH and have had some success using Autoglym Tar etc Remover. But some won't budge. I can loosen some with my finger nail (I know that's not a great idea but it shows that they are removable) and then the Autoglym removes the rest...........sometimes! I'm beginning to suspect that some are air-borne pollution as we appear to have them on our bins and on some plant leaves. Anyone have an infallible suggestion?