you may try a pressure washer. i bought a corvair once that had been parked under a tree for a while and had much dried bird poop on it. i was trying to remove the dried droppings at the quarter carwash(a long time ago). suddenly the paint started to bubble and i could peel off strips. when i got most of the top paint off i realized the car had been painted without a drop of prep, except for the center of the drivers door that had a large ding in it.

i love my dog's 1965 suburban.