There are two separate issues to address, so its should be a two step process. Species of ply makes a difference as well. But, you need something to soak into the wood and seal as much of the wood surface as you can. This should be a small particle size, less viscous substance. An epoxy primer would be my first choice, second choice would be a thinned polyester. Then you need something to bond to the sealed wood surface which provides surface and UV protection. Once the wood is sealed, you have many more options on the surface. A single step process will result in a lot of material on the surface, but very little penetration into the wood. My experience has been poly works, but I get better penetration with epoxy (one of the few areas I use epoxy in).