Further to the posts from Mark Singleton and Craig Smith on de-rusting
discs, UK readers might like to know that Evapo-rust is available from
Frost, who are UK restoration equipment suppliers, mostly focussed on
cars:

http://www.frost.co.uk/evapo-rust-super-safe-rust-remover-946ml-us-quart.html 

I have used Evapo-rust for many years on steel parts for organettes,
small organs and player pianos, although I have not yet need to try it
on discs.  I sold it for a while when I had the supplies business but
the importers then started selling their own product, which was similar,
that was then taken off the market a couple of years ago when Frost
started selling the original product.  I always keep some in stock and
can vouch for everything Craig has said.

Regards,
John Farmer, UK