A covering of snow foam left to dwell would probably be a good start.
Follow that with something like megs apc at the correct concentration, note this will not only strip the crud but any protection the paintwork had historically (if there ever was any

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After the apc you would probably do well to clay the whole car, wash with ordinary shampoo then work up through polish, sealant wax etc.
I'm sure there'll be some experts along shortly though. Get some before and after shots