The interior surface of wooden windows takes a surprising amount of stress. Damaging levels of ultra-violet light shine through the glass, wide swings in temperature occur, and many windows develop at least a little bit of condensation during winter, wetting the wood in the process. The bottom line here is that even though the inside of wooden windows is an interior surface, it’s best coated with a film-forming exterior finish. As much as I like tung oil for many applications, I wouldn’t use it on windows. Conventional waterbased urethane isn’t great either, since most formulations don’t stand up to UV rays.
- I’ve had good results using Minwax Helmsman exterior urethane on interior wooden window surfaces:
- it’s easy to use,
- dries virtually clear,
- and forms a tough film yet creates a smooth finish.
- Remember to sand the wood lightly with 240-grit sandpaper or a fine 3M rubbing pad after the first coat has dried.
- Sikkens Cetol works very well on windows, but all versions are some shade of golden or brown colour.
- Also – and this is important – I’d wait until warm weather in the spring before finishing your windows. Even though your room might be cozy during winter, the wood of the window is quite likely to be too cool for any finish to dry properly.
When it warms up enough for finishing, you’ll get best results if you sand back to bare wood first. A detail sander is the perfect tool to use. As a final step, use a razor blade scraper to remove any finish that got on the glass.