go to your local motorcycle shop and ask them for a product called muckoff. water this down 50/50 and spay on. wait 2 mins and brush into the stain then wash off with water. All done and will not damage any aluminium, paint, fibregalss or platic that it gets on.
Fenwicks yes. Would not use anything else. If the marks have been there a long time and are really stuborn, use neat Fenwicks and wash off immediatlly.
Like others i say AUTOGLYM caravan cleaner,Its the BEST and you wont use anything else when you have used it,Just spray it on and wash it off,its simple with good results,
i searched this once upon a time and for the life of me i cannot remember the source of this info, for normal black marks i use wet wipes, cheap and easy, for the stubborn, fenwicks.