You do not have to believe me, but, a long time ago, in the Hasselblad factory, there was a kind of a rotating cylinder (like a concrete mill in miniature, a tumbler as Archphoto told?) with hard rice grains in (the one they eat in China) and the tiny parts got polished during a long time in it. I wonder if some kind of rice powder might do the job for you...

Good luck,

Philippe