[clairvoyant]

I clayed my car once a year (usually late May/ earlyJune).
So far nothing, my clay was clean. My car was far too clean for going through the harsh winter here.
Where as with a white car (see the attached photo), the rail dust is very noticeable.
How do you spot rail dust on black cars?
Perhaps, I didn't clay it properly....but then I followed the instructions on the sticky. Even Picus showed me how. Thank You

[TOGWT]

After washing and drying your vehicle place a plastic sandwich bag over your hand or fingers, gently pass over the surface. Does the surface feel silky smooth or rough and gritty? If you felt roughness and/or grit, your surface is contaminated

[Tzu]

I hate those spots. Is "Rail Dust" the correct term for it? What exactly causes it?

It usually comes off with clay, but its much easier with IP or AIO. With the PC Orbital, its a breeze.

[rvladescu]

I use and recommend to use ironX to remove this stuff off your car before buffing as you could scratch your paint with those particles of iron