You have quite a bit of options really.
It depends on what type of surface you want. Matte, semi gloss, gloss?
In the past I've used mod podge. But it left a bit more of a sheen than I like now.
You can use polyurethane, stone sealer, concrete sealer, epoxy, pretty much anything you can find that waterproofs will work. I'd look for something made for outdoor use as usually they are more uv resistant and wont yellow. At least not as readily as others.
-Brandon