MMan....
Not sure what kind of look you want, but I've used 400 paper with break free and gotten very good results on the flats. Give the flats a coat of oil, then as bjg-1911 said, go in one direction with one straight stroke several times, lightly, until you get the desired results. Wipe off the oil, polish the flats with Flitz, give it a light wipe down with oil and you'll have a nice even sheen. I've used this method to remove slide stop scratches on the slide and it works like a charm.
Regards,
Sam