It wasn't direct overspray, but the secondary fog of half dried paint aerosol floating around the room. It was rattle-can paint, not epoxy primer or some good stuff. The car also has one heck of a coat of wax on it, so the overspray didn't have the best base to stick to.
I tried the claybar on water spots on my focus today, it worked pretty good and got about 80% off the first pass. It sucks having to park this car on the street where it gets hit by sprinklers twice a day.