I refinished a rifle stock with spar urethane. Why? The idea was to produce a waterproof stock, for use on a go-anywhere utility rifle. While there are good synthetic stocks in abundance, these days, that are impervious to moisture, I already had the wooden stock and a little bit of time on my hands. Also, I kinda like wood.
You can apply spar varnish over a factory varnish finish, just roughen up the original finish a little bit with fine sandpaper--220 grit or so. Be careful to seal the stock completely, with plentiful spar varnish inside as well as outside, and take off the buttplate and seal the wood under there, as well.
Using multiple thin coats, build up a thick protective film that encapsulates the wood and keeps moisture from reaching it. Does it work? Sailors have used this stuff for years to seal wood against the elements and it seems to work for them.
How does it look? You be the judge. I used satin finish varnish for less shine afield, and I think it looks nice. Odd, but nice. The thickness of the finish reminds me a little of bowling pins, but otherwise it looks all right. We'll see how it looks after being knocked about outdoors for a while.
When I was already more than half done with this project, it occurred to me that maybe I was going about this wrong. Many years ago, the best-finished stocks had finishes of deeply soaked-in linseed oil. The oil penetrated for some distance into the wood and created a moisture barrier. Here is a description from a 1920 book:
Finally the gunstock comes from the machines completely shaped and with every cut and every hole exactly like those of every one of its mates and stacked with twenty five to fifty others in a rack on an electric truck it goes for a bath of several hours in hot linseed oil. After it has absorbed about quarter of a pound of oil it goes to the staining room...
It was only after this process gave way to more modern (quicker and cheaper) finishing methods that people started complaining about their stocks misbehaving in wet weather. At least, I do not recall seeing, in old time shooting literature, concern expressed about the problem. It could be the old timers had a problem and didn't know it. Or it could be there was mention I overlooked, but it's my impression that the fit of the stock to the metal shifting around in wet weather is a modern concern.
Very likely the problem here is the modern finishing method that leaves the wood inside the stock raw and unfinished and puts a pretty coating on the outside. This is the way the factories have finished most wooden stocked rifles for the last fifty years or more.
It seems to me that the oil soak method likely works about as well as encapsulating the wood in a plastic based spar varnish, and looks more traditional, to boot. But, as I say, I didn't think of that in time, this time.