some sort of protection over exterior paint?
Put a thinned coat of a Spar varnish on it, lightly sand- do your lettering, then finish with a coat or two more of the spar.
It is an oil finish- so be aware that it will amber- more over time.
This will change your lettering colors a bit too- blues will look greenish, etc. But it will hold up the best.
It will bring out the wood tones very nice.
"It's better to come here with questions before you screw up than to come here after and ask how to fix them."- JS