Rust

["Tim Bunton" <tbunton@zzzzzzzzzzzzzzzzzz>]

Clive, Cold Gal is a great way to stop the metal below from corroding.
The only potential concern I see is that the zinc will now be corroding
preferentially, right at the stone interface.  Although zinc oxide does
not have the characteristic vivid reddish-brown color of iron oxide, it
does have a chalky yellow-white color, which could result in some degree
of color transfer to the stone.  I definitely think you will be better
off than you were, though.  You might consider painting the steel with
Cold Gal and then the contact area on the stone with the polyurethane or
a stone sealer to prevent absorption of any oxides into the stone
itself.

Tim Bunton