I have for a number of years used a vibratory tumbler to clean and
polish small metal parts when rebuilding.  It works quite well.  I use
a True Square brand tumbler that is sold through W. W. Grainger.  The
tumbler compound I use is the one they call for in polishing.  It is
really just dry, ground up corn cobs with some mineral polishing agent.
It removes very little metal and is more of a polishing compound.
I leave the screws in the tumbler for several hours at a time and check
on the progress.  It removes scale, light rust and polishes very
nicely.

They sell many different grit compounds that can remove increasing
amounts of metal during the tumbling process.  But in the case of
screws I wanted to make sure the threads were not in any way reduced
or dulled.

Before I tumble the screws, I put them is some form of paint remover to
remove any paint, dirt or grease.  After a rinse I dry the screws and
start the tumbling.  If the screws are really rusty, I put them in
dilute Hydrochloric acid (hardware stores sell it as muriatic acid) and
watch them closely.  The acid does a great job removing rust but will
pit the metal if you leave it in too long or use too strong an acid.
It will also remove plating.  I do my own nickel plating so this is not
a problem.  (Note: even with dilute muriatic acid, for safety sake
always further dilute it by adding the acid to the water not the other
way around or it can boil up on you!)

Bob Humes