I've made a sign that will be placed outside.  It's just made out of a 2x12.  I will sand it to a finish with 220 grit so it shouldn't hold water as much as regular wood but I want to coat it with something that will last.
I thought that a marine polyurethane or marine lacquer might be the answer.  I've painted the lettering with an oil based Rustoleum enamel and I'm wondering if I can coat over that with the marine coating.

Do any of you have a wood boat that you've used something on that would work?  I did paint a bench using some deck paint that was alleged to last and it did, however it's orange and I want clear.