If you're talking about SEALING the stock versus "finishing" the stock, you'll want something to go into the wood, not just on the surface.
I've managed to seal difficult pieces of wood by stripping them then submersing them in a mixture of 50-50 Naptha and spar varnish. After doing so once, wipe it off completely, let it dry, and do it one more time. It helps if you let the stock dry out and heat up in a hot attic just before you do it. Then apply your finish. I'll guarantee that stock will never move even exposed to constant moisture.
