We get some similar issues in the rust belt. Driving around all winter
in the salt spray leads to corrosion on the slide pins. This means the
piston side of the rotor takes all the wear, the opposite side gets
minimal wear. Then let the vehicle sit for a few days, you get heavy
rust pitting. In the winter, you never get to really hit the brakes hard
so you never really wear off the rust pitting. The fix is to lube or
replace the caliper mounting pins.

As for turning brake rotors, I've not had good results for high
performance vehicles. We usually see rotor warpage within a few thousand
miles. For track usage I'd go with new ones. I know, lotsa $$$ but it's
the only way to get the safety factor back where it should be. BTDT: ex
crew chief on a Porsche GT-3 Cup car.



David Lesher wrote:
A friend's Audi sedan made some funky noises while braking.

Pulling the solid, one-piece, rear rotor revealed why. The outer surface
was fine, but the inner one showed the usual ~~smooth surface, and
several patches where the surface was anything but. It was as if the
surface was a plating or veneer....and flaked off. Underneath, the steel
was rusty and visibly porous. This was in several spots from dime to
quarter sized.

While none of us are 8-5/40Hr auto mechanics, between us we have multiple
engineering degrees, and many decades fixing our own cars; we've never
seen anything like such.

I'd assumed rotors are cast then turned, for both size and
thickness. True? Are they hardened at some point? What could explain
pores in such?