Although we haven't actually tried this ourselves, a lot of people swear by WD-40 (amzn.to/2HHt5fG). It won't damage paintwork and can be used to remove a wide range of muck off cars. I'd also recommend using a pressure washer first, then the WD-40. Finally, polishing and waxing your car will likely make it much easier to remove next time you want to wash it. We advise using a polish like Poorboys White Diamond (amzn.to/2VTw3rh), followed by a sealant like Bilt Hamber Double Speed-Wax (amzn.to/2WbKNBa). Wax like the Bilt Hammer will take a bit of effort to buff but the end result is worth the work.