Which finish should I use on window casements?

I am in the process of refinishing wooden casement window frames. What kind of finish should I use? Is there a difference between spar varnish and marine varnish? Would any products hold up better than these?
—Leon Wallwey, Midland, Mich.

Neither spar varnish nor marine varnish will work best on window frames, although for different reasons, Leon. Window glass allows enough UV light through to degrade the UV inhibitors in most oil-based spar varnishes. Once the UV light breaks down the wood fibers, the wood releases the finish and begins to discolor and decay.

Many true marine varnishes use UV-resistant phenolic resins and tung oil, but they leave a flexible, soft finish not meant for interior use. Marine varnishes tend to be much more expensive and harder to find than spar or interior film finishes.

For an interior finish that uses UV-resistant phenolic resins and tung oil, one option is Waterlox Original Sealer and Finish. At $27 a quart at the time of this writing, it's still not cheap. But you also can buy it in 2-oz. containers for small touch-up jobs. It won't eliminate this chore, but refinishing your window frames won't become an annual ritual.

Whichever replacement finish you choose needs to be applied to sound wood. So sand and restain sun- and water-damaged areas before refinishing.