Epoxy sign

I carved a sign for a customer and they want the letters filled with solid white epoxy and stain over the rest, After that they want the whole sign sealed. I am having trouble with sealing. I tested minwax indoor/outdoor urethane on anther test board but it turned the letters a brownish color. What can I use to cover or seal the entire sign?
I am using east coast epoxy

For outdoor signs, I use Varathane which is oil based…
For indoor signs I use a water based poly seal.
I spray both and get an excellent results.

Which one of the outdoor Varathane do you use?

Sorry, I should have been more specific.
Its Spar Urethane, made by Varathane
I buy the spray cans.