Old etchings

[slimfandango]

they develop that strange 'dotted rusty' pattern. Any way to remove this?

[Skids]

That's pitting corrosion and in its early stages it can be removed with rust remover or rust neutraliser (eg Jenolite Jelly) without too much damage to the surface. If its allowed to continue for a while, the surface will be very damaged and look like the side of a house in a Bosnian war zone.

[mrs.pudding]

Or it could be 'foxing'.  This can be removed by a specialist but not a cheap process I'm afraid!