Originally Posted by
bkkboy
The paint may have discoloured over the years. But there appears to be dirt embedded in the surface of the paint. If it was dirt stuck to the surface I would have removed it by now.
Decades ago when I watched MotorWeek on PBS, they had a segment on exterior car cleaning. Usually a wash would work, if not, wax. For more stubborn dirt, after a careful wash, they showed dragging a block of clay(?) along the paint, slowly and with moderate pressure, only in one direction, I think front to back, and that would grab more embedded dirt out of the surface, evidently. I've never used. Do some looking online about paint cleaning methods for full-tilt, "detailing" level cleaning, and if you can't find, call around to exterior paint detailers and suppliers to them.
EDIT: Quick search by me, I've not read it yet...
https://www.caranddriver.com/car-acc...-clay-bar-car/