We've seen great examples using stealwool to polish, but in our test sanding brought out PLAs white color. Polishing compound (Brasso) can also be used to get a patina look on the model.
Use 100 microns or higher to achive a smooth finish. Lower resolutions will take more time to smooth out. The longer the model tumbles, the more likely parts will brake.
Brass screws wears down after about 120 hours of tumbling. Screws grind and then the metal particles stick to parts, giving them a brilliant shine. We recommend using new screws to help maintain a consistent polish.