Originally Posted by tothevancave
he hypothesized the coating might be for shipping but admitted it was just a guess. .
I had a set of Powerstop rotors that rusted horribly in six months. They were drilled and slotted, and the rust clogged the holes and slots. The interior of the cooling fins were solid rust too. I called Powerstop and discussed all the rust and hypothesized that the metallurgy was such that there was little to no corrosion protection cast in, so bad rust was inevitable, and they agreed that some blanks cast in China had less quality control over the metallurgy. The sent me a new set made in Germany they said solved the rust issue. I couldn't wait for them though and replaced the slotted ones with locally sourced ones, so I've never run the new one's and can't report on rusting. I suspect the "coating" your hearing about may be a rust preventative of some sort with no impact on braking or wear. Once in a while I'll get a brake caliper that is coated with something and doesnt rust, but most of them are bare steel and even if painted, they rust. I've tried ordering coated ones, but it's random, they all seem to have the same part number.