I will have to agree with the previous post from nj1266 concerning the majority of warped rotor issues as being the cause of deposits. There is an excellent study done at Uppsala University on the issue of break squeal, but is useful reading to better understand the metalurgical dynamics of a breaking system.
http://www.angstrom.uu.se/materials/...n%20Thesis.pdf
This is some heavy reading, but supports how the surface intergrity will change in breaking applications. see comment below:
nj1266 "...Let me start by saying that machining rotors is BULL CRAP. Rotors do not warp. It is an old wife's tale that needs to be debunked. I have been racing cars for 5 years and I have not seen a warpped rotors yet. Here is what happens: Your brake pads get overheated for various reasons and then they transfer brake material unevenly to the rotors. When the brakes cool down the brake material sticks to the rotor. When you drive and hit the brakes you get the wobbly impression that is very common to all of you. Machining the rotors destroys their structural integrity. What needs to be done is to sand the excess brake material off the rotors and use NEW brake pads. In essence, the brake pads are the problem and not the rotors..."