I have no idea if this will even remotely work, but I'll throw it out there... I would not try this with one of your knobs without testing it on something that you don't mind throwing away, but if you know someone that polishes brass for reloading ammunition, try dropping it in one of the drum type machines (no chemicals, just the dry abrasive) and let it run for a little while. I would guess if it turns out nice, you would have to seal it with some sort of clear-coat to protect it when done. I guess the ones for polishing stones are about the same, but I don't know the grit levels of abrasives used in them.
EDIT--brain fart when I posted--the real name of the machine is a "tumbler"--used for polishing brass, jewelry and stones.