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I'd try mineral spirits first. Something mild, or tar remover. Acetone should work, but I would test in a hidden place first. Some paint will react badly to it. Consider it last resort.
If it is water based latex then mineral spirits won't work. I tried that on Sunday. 3M General Purpose Adhesive remover did work with some elbow grease. Then I tried a clay bar and that worked even better for the white paint on my car. Don't you hate it when some trucks purposely leave their paints loose to spill rather than take to the recycle center...
I have used a clay bar before. It was like magic. In fact the stuff I used was called magic somthing. I think it was magic clay bar. Anyways it worked great. The paint was even on there for like a week before I got to it.