^ +1
I have a few of these rail dusts near the corner by the headlights, here's a link that might help you remove them, http://www.detailedimage.com/Ask-a-P...nd-a-clay-bar/

So I read around for more info, seems like these stuff works really well. You'll want to wash the car and keep the surface cool, spray on the iron X and agitate it a little with a sponge/microfiber cloth, and let it sit for 5 minutes or so. Then rinse it off as much as you can and clay bar. After that you can do your polish, waxing, sealant, etc. I'll do this after winter for sure!