My old piano tuner, who died at the age of 93 a few years ago, used
to clean strings by rubbing up and down them with an old fashioned ink
eraser.  That really only provides a temporary fix as they are likely
to corrode again with damp and time.

The other problem is that, in my experience, steel piano wire is
normally plated with another metal and rubbing the strings with an
abrasive is likely to remove or damage that plating, thus hastening
further corrosion.  Another, less drastic, possibility would be to
polish the strings using a magic wadding type polish such as Brasso
or Duraglit, and follow that up with a soft cloth.

My former piano teacher taught me a lot about piano restoration and
actually served his apprenticeship with Tomasso's Barrel Piano factory
in Leeds, Yorkshire, England !

If Joan Tomasso would care to contact me I will give her the address
of an MBSGB member in Yorkshire who is a mine of information about the
English branch of the Tomasso family.

David Snelling - Isle of Man.