Originally Posted by JCL

Don't grind the rotors with a power tool. If there is surface rust on them, you can use some emery cloth to take it off, but you don't want to do anything that changes the thickness of the rotor or you will get pulsing in the brake pedal.

We used to turn rotors on a special lathe, and then reface them, but not so much any more. BMW rotors are not designed to be turned, they don't have as much extra metal there (which results in less un-sprung weight). If the rotor surface condition is reasonable, there is no lip at the edge, and they are not down to the minimum thickness (stamped on the back of the rotor) you can reuse them. They will not stop as well, or resist fade as well, as new OE rotors, but may end up still being better than cheap replacement no name rotors.