How to weatherproof outdoor painted plywood sign

[spock]

So I picked up a really cool vintage hand painted plywood sign that I am going to use as the back of a bench I am making. I do not know if the painted graphics are oil or acrylic and not sure how to tell. What is the best way to treat it so as to help preserve it for as long as possible. It was originally outdoors and still looks great - I am hoping to keep it that way. The sign is 2' by 4'.