Originally Posted by
Molly
Spar varnishes are good choices for gun stocks. I've used them, and they work well. They were originally made to protect the wooden spars of ocean-going sailing ships, so you can be sure they are weather and water resistant. Any polyurethane finish will do well too, but chose a polymer based on "IPDI" (isophorone diisocyanate) instead of "TDI" (toluene diisocyanate) if you can. They will both give you great protective films, but the IPDI polymers are FAR more resistant to UV degradation from sunlight.
NO oil based finish (linseed, tung, etc) will give really good moisture resistance. Adding a polyurethane to an oil finish will upgrade their water resistance in proportion to the percentage of polyurethane in the vehicle.
Acrylics will also generally improve oil finishes, but not always. It depends on the choice of acrylic: Some are tough, some are brittle. Some are durable, some aren't. It all depends on what problem the chemist was trying to solve when he selected the acrylic.
You can take the preceeding to the bank: I am a retired polymer chemist with about 45 years in the paint and coatings industry, and know whereof I speak.