100% sure it’s rail / brake dust. All vehicles have it, and will get it. Ya just see it on white cars.
Yes clay bars work, but do yourself a huge favor and buy this stuff:
Remarkably quick and effective iron remover for the exterior paint and wheels. Safely removes stubborn iron deposits.
www.detailedimage.com
it smells bad. I mean really bad, like rotten egg fart bad, lol. But it will remove 95% of iron (rail dust) from your paint. It turns purple You will be shocked at the amount of iron on your vehicle.
further remove bonded contaminants from your paint,with this instead of a clay bar:
Replaces the need for a clay bar while saving you time. The fastest way to safely deep clean your paint.
www.detailedimage.com
you attach this to a polisher. Will save your hands, and lots of time.
this is the first step before polishing that will give you a nice smooth surface.
but seriously if you don’t want to go down the rabbit hole of detailing, just spray the first chemical on your paint. Let it sit and gently scrub with a wash sponge. Rinse, and enjoy no more “rust” on your car.