do you want something clear, semi trans parent , or solid color?
And what kind of wood is it.
if the sign is white oak, cedar, or red wood then you can actually go natural with no sealer, especially if its covered with an awning.
If you want a solid color like paint then any satin or semi gloss paint would really work just fine. some say you should put a sealer on first if its up on a wall and and protected to some degree than the paint is enough.
The problem with clear coats is the sun. If you want clear the best thing for out doors is a val spar poly with a uv protectant.
The wood has to be fully dried before any of these are done. that is very important.
