Street Striping Paint
Apparently one of the trucks applying white paint to the streets dumped a bunch of paint on the road just after I got tube fenders on my 2014 JKUR. My right side tires went through the glop of paint and it got all over the inside of the fenders, on the rock sliders, all over the bottom, on the tub and even on the spare. Like the OP, I used everything in my garage that I could find to get the stuff off (scrubbed for at least a couple of hours with mixed results). Decided to take the Jeep to the squirt it yourself car wash the next day and noticed that the high pressure wash wand was taking the white paint off but was not affecting my tub paint, the tires, wheels, rock sliders or fenders. It didn't get every little splatter off but you have to look carefully to see any. If I'd taken it to the car wash the day it happened, I'm sure I could have gotten it all off.