Dennis,
I used to do gold leaf lettering and stripes on firetrucks.
I know that Dupont made a clear coat that was used on firetrucks. This clear coating withstood extreme heat and had great chemical resistance.
I haven't used Dupont products in a few years so I cannot help you with the product name, part number, offer a better description or offer more information.
My only suggestion would be to do a search for companies who build and paint firetrucks. I'm sure they will tell you what is currently being used, if they still use a clear coat over colored paint.
The products could still be available and probably with improvment.
Might be worth the shot.
I hope this helps.
Charles