While I haven't actually tried this, I have heard that you can run
a piece of fine string or dental floss charged with, say, tripoli
between the spaces; or maybe a thin strip cut from fine sanding
cloth or film.
Janet - this is called thrumming and it is described in Oppi
Untracht’s book “Jewelry, Concepts and Technology”, and he says
"Thrum is a weaving term meaning any mass of fibers, thread, or
loose, coarse yarn. Thrumming in metalwork is a form of hand
polishing parts of a work that are otherwise inaccessible by the use
of a length of coarse, soft threads or thrum charged with an
abrasive. Thrumming thread is available in coarse, medium and fine
grades. They can be used in bunches or singly, depending on the need.
One end of the thrum is tied to any rigid member, held tight, and
charged with a bar of tripoli or rouge for rubbing it along its
length. Only one type of polishing compound should be used on a
particular set of thrums. The free end is then passed through an
opening or depression in the work and is then held tautly with the
left hand. The work is moved up and back on the thrum with the right
hand, changing its angle of contact as necessary to reach the desired
places. Additional abrasive is added when needed. Thrums work
quickly, and the work must be examined often. The thrum is withdrawn
when the work is finished." Copied from Oppi Untracht’s book.
To this, I’d like to add that I’ve tried this method of polishing
pierced metal and can say, you really must watch closely as you can
easily “cut” the metal if you use a hard fine string pulled very
taut for a thrum. I’ve had best results using several strands of
very soft cotton yarn (like is used for knitting). But if you don’t
watch closely, you can truly cut ridges inside the pierced areas
from rubbing it on the taut held yarn (string). But if done
carefully, you can get a lovely polish on otherwise unreachable
areas or difficult to reach areas.
I suggest you try this method on a scrap piece of pierced metal
first (not your final creation) to see how it works and how quickly
it “cuts”. I’ve also used thin strips of very fine sanding paper
rolled into pencil thin rolls and inserted in the cut out places.
But it doesn’t work as well for me as the soft cotton thread rubbed
with polishing compounds.