I used the method described by Alan for a set of bars, worked a treat.
For deeper rust on a Cr-Mo stem I have, I got Hammerite Gel rust remover.
Put it on a millimetre or so thick and leave until you can see the rust blacken underneath, then wire brush the rust off.
Repeat if necessary and the rust is gone, just clean bare metal left that you can protect with polish/wax/lacquer etc.