Wood windows with a natural finish on the inside look great when they're new, but moisture, temperature swings and harsh sunlight eventually make the varnish fade and flake away, especially along the bottom of the window sash and on the sill. It looks like an intimidating job, but usually the only part of the window that needs work is along the bottom—the rest of the sash and trim can just be lightly sanded and recoated.
The first step is to match the stain and finish. The easiest way to do this is to take a piece of the trim to a paint store to have it matched (Photo 1).
Scrape any worn varnish off (Photo 2), then sand with 120- or 150-grit sandpaper. In partially damaged areas, feather the sanding into the undamaged areas, then lightly sand the rest of the piece to prepare it for a new finish (Photo 3). You don't need to sand and recoat the entire window if only a few pieces need it—just stop at an edge or corner.
Stain the wood (Photo 4) and apply two coats of outdoor-grade finish. If the gloss doesn't match the older varnish, buff it lightly with extra-fine steel wool or pad to dull it.