Rosemary:  I have had good success cleaning steel discs as well as
steel tools using Evapo-Rust and other "chelation" type products
in a bath.  If you leave the disc in the solution too long it will
remove the printed design and title, so leave it in only long enough
to remove the rust.

The solution bonds with the rust which literally falls off with light
brushing.  But first, test for the length of time to leave the disc
in the solution.

Bill Wineburgh
Succasunna, New Jersey
http://antiquemusicbox.us/