I have cleaned billions and billions of screws.  The easiest way is
a wire wheel on a grinder appliance, or just on a motor you pulled out
of something.  I also use this to clean player lever linkage rods and
anything else that is metal and is not plated.  Plated items are
polished on a cloth wheel.

The are two useful wire wheels: the coarse and the fine; not extremely
fine, however, and not brass.  Wheels must be really good quality or
they throw wires all over the place and destroy themselves within a
week or two.  I usually use one for a year or more and I clean millions
of screws in that time.

Use pliers to hold the screw or you will file down your hands.  If your
metal to be cleaned is really rusty, use the coarse wire (not the kind
with twisted ropelike braids -- that is way too much overkill).  For
most piano and player screws the fine works well  I believe the wires
are about .010".  Either of these can be bought at Grainger for about
$30-$40 each.

The tumblers for screws you remember _are_ the rock tumblers you used
to use as a child, still being made by the same company.  I use them
when I have lots of screws to clean and I don't have small lots that
must be kept together.  When I tumble I just put in the screws with no
media added at all.  They tumble for 4-6 hours and I check them, then
tumble longer if needed.  Do not tumble machine screws as the tumbling
will dull the threads and make them unusable.

When they are all clean, pour out the screws into a flour sifter type
strainer and let the black dust fall out.  You may blow them out with
a compressor nozzle.  You may also rinse them several times with hot
water and dry them, to prevent rust, in a pan lined with rag or paper
towels in an oven at 200 deg. F.  You may also do the final rinse in
alcohol or lacquer thinner to get them dry faster with no rust or
heating.  Don't use thinners if you do the oven thing!

D.L. Bullock    St. Louis
www.pianoworld.us