There's a type of epoxy that they do counter tops and clock
coating with. That can be mixed in larger volumes and used in
a bottle with a syringe needle to do large areas such as a big
sign might have.
It's a softer epoxy, but that may not matter on something like a sign...
I never tried it myself. I do know that different colors have different
solvents in them and that can affect the hardness with that sort
of epoxy. But it does work very well enough to warrant some trialing & erroring....
John