I’m sure a few years back there was a guy on here cutting down damaged signs. re-bordering them and then polishing with Cerium Oxide (he called it something else like, ‘powder pink’…).
I’ve used the Cerium Oxide with a Random Orbital Polisher, and it does gloss up the sign. But it does not remove water/acid damage. The real question is regards the Silicon Carbide discs as if you watch the videos, they are quite aggressive and remove material, but are the only way of getting into removing that water/acid damage that causes the dull matt finish.
Unlike glass which is the same all the way through, enamel signs are ‘layered’ with differing colours, never being sure how thick each colour layer is. My fear with the Silicon Carbide discs is that you could accidently go too far and remove a coloured layer.
What they can do on damaged glass., followed by the polish, is simply amazing, just not sure about using on enamels…