I'll go a step further and recommend a specific brand. Minwax Helmsman Spar Urethane. Wallyworld ought to have it. If not, Lowes or Home Depot does. Spray very thin coats and build up the finish gradually. It'll eventually peel off if you only use a couple of heavy coats. Found that out the hard way.I use the stuff on my wood arrows too.

After giving it a try, I really like using Tru-Oil gunstock finish on wood risers and overlays, but poly is better on the glass.