Any advice gratefully received. Thanks.

In my experience, all domestic, normal enamel white gloss paint yellows over time. If you are serious about this you should consider automotive paints and sprayed NOT brushed on. Find an independent car resprayer who shows an interest.
Restoring seems to be an obsession for some colectors, and I don't see the point when the sign is badly damaged. The sign ceases to be original.
Fred Kite

Thanks for the advice.  That's what I was thinking.  The only thing that bothers me slightly is that most automotive paints are acrylic based.

I agree with you about restoration, and I'm not keen on restored signs either.  However, sometimes there really is no choice, other than scraping the sign, and that would be a waste.  If the restoration is done to a high standard, then it's quite acceptable.  I can do the artwork to a high standard, I just need to know the best materials to use.  I've worked mostly with showcards, so I know what to do with them, but enamel is a different proposition.  I could do with a chat with Steve Burton!