This article is from the Toyota Tundra FAQ, by Steve Yee with numerous contributions by others.
These brown spots on your paint are imbedded metallic particles that has gone
through the clearcoat and has imbedded itself underneath the clearcoat.
Commonly called "Rail Dust", the particles can be removed by using a clay bar.
Some severe cases can be cured by using a spray of oxalic acid, but that can
damage plastic parts. According to DaimlerChrysler Corporation, clay bar
treatments are the only recommended solution to remove rail dust on all their
vehicles, including the Mercedes-Benz Line. Toyota recommends claying the
vehicle before waxing them in their Lexus recondtioned used car certification
program.
 
Continue to: