Originally Posted by TBatt

Here is what I use to resurface the rotors so they can be re-bedded. Works like a charm. Best if the rotors are removed but you can use this with the rotors still on the car. You will have to remove the calipers to get to the back surface if you leave them on the car. You should scuff the pad surface too using 80 grit sandpaper if you plan on re-using the old pads.

Brush Research Flex-Hone For Rotors: Amazon.com: Industrial & Scientific

Rotors don't really "warp" they build up an uneven layer of pad material and this is what causes vibration when braking.