I have to agree with John Oakes' posting except for using steel (wire)
wool on the bass strings.  The steel wool gets trapped in the copper
windings and that's not good.  The oxidation on the copper windings has
little, if any, effect on the tone, it's the dirt caught between the
windings and the core wire that's the problem.  John mentions tilting
the piano on its back, which is a wonderful way to work on the piano,
but most folks don't have a piano tilter handy.

As for polishing the plain strings, you can get a model train track
cleaner block from a hobby shop, it will do the trick too.  It's
basically an eraser with fine grit in it.  You could also polish them
with 2000 grit wet/dry sandpaper, but rust pits in plain piano wire
will provide a nice breaking point.

Typically, the problem with piano strings shows up mostly in the bass
wound strings.  Yes, a lot of dirt does fly around!  If I were doing
the bass strings on a grand, I would put something like a sheet over
the strings so the one I'm working on doesn't dump dirt into the piano.
I should have mentioned that in my first post; sorry.

Do be prepared for the bass going out of tune for a bit after you've
done all that work.  One grand I did took about three tunings before
it settled down.  And yes, the bass was greatly improved, but the piano
action wasn't!

That piano needs a full rebuilding, but it's just a "lobby piano"
at the local theatre, and they can get a better one donated cheaper
than fixing this one.  (Actually, the theatre has one waiting in the
wings for me to touch up the case before putting it in the lobby; if
I weren't so busy restoring the theatre organ for them, I'd have it
done by now!).

Well, enough of being on the 'Net for a bit; now I have to go dig up
my septic tank!  Yeech!

David Dewey
Oroville, California