Maybe bead blast? You might try:
http://www.chem-pak.com/plastic/perfix.htm
They supply spray coatings for various thermoplastics. They have
specific grades of matte and eggshell finishes. It does work pretty
well, and you can request a sample.
--
Billy Hiebert
HIEBERT SCULPTURE WORKS
Small Part Injection Molding
http://www.hieberts.com

Spencer wrote:

Hi folks,

I'm a mediocre self-taught machinist, but I can usually make parts
that *function* as I would like for prototypes, etc. For production
work, I use local machine shops. My favorite shop went out of
business a couple of years ago, and now I'm stuck. They used to make
some cylindrical parts for me turned from black delrin. They had a
very nice matte finish -- almost as if it had been etched or tumbled,
but I don't *think* that was the case. I hadn't needed any more of
these parts for awhile, but now that I do, the other shops tell me
they don't know how to duplicate the finish. The owner of the first
shop is no longer available.

So, does anyone know how to do this? I'd also like to get the same
matte finish on turned black ABS parts. My experiments with different
grits of SiC sandpaper, with and without oil or water, have gotten me
a little close, but still not the lovely, even, original finish. I
still get this little bit of whitishness (sp?)to the surface, as well
as little "hairs" of plastic. Also, the sandpaper loads up pretty
fast.

For the ABS, I've been thinking about using a very dilute solution of
acetone and water or acetone and alcohol after the sanding. Pure
acetone dissolves ABS quickly, leaving a very shiny surface after it
evaporates. My thought is that the dilute solution might wipe out the
hairs without dissolving the bulk material much.

Thanks very much for any advice you can provide.

Spencer