Window and casing finish

[Cody Sibell]

Hello,

I need to finish some new windows and casings. Unfortunately, they are already installed and thus can't be finished while laying flat. My only finishing experience has been kitchen cabinets, which I did using Lockwood dyes, Seal-A-Cell and Armor-Seal. I have a feeling that a polyurethane varnish is not ideal for this application. I was thinking that either something like Sutherland Welles Polymerized Tung Oil or an alkyd varnish (though aside from the Minwax Fast-Dry, this seems to be like finding a unicorn) would be better. The wood type is red oak.

Thanks!

[Edward Dyas]

Most polyurethane varnishes are for interior use. You wouldn't want to use an interior varnish for windows even in the inside of the house. The main difference between an interior varnish and an exterior one is the interior varnish dries rock hard. Then on an exterior application the wood expands and contracts a great deal more and the rock hard finish cracks from the stress. Then water gets under the finish and it flakes off. An exterior varnish is formulated to be more flexible so that it doesn't crack as the wood expands. Then in applications where water is frequent it needs to be more flexible so a marine grade spar varnish is normally used. Now a polymerized tung oil works as well as a spar varnish in that application. Tung oil was originally used to finish hulls on boats. Then having red oak you need to be especially diligent about maintenance. If the finish is ever allowed to crack where water can get under it red oak is prone to turn black and rots very easily. White oak would have been a better choice of wood. While it looks nearly alike the wood is suitable for wet locations. In some parts of the country it is used on the exterior of a house and not finished at all. It's just allowed to turn gray and age.

[Cody Sibell]

Thanks! Would spar varnish or tung oil be better? Do you recommend a particular spar varnish product? Is there a functional difference between polymerized tung oil and raw tung oil, other than drying time?

Unfortunately, white oak was not an option from the manufacturer - the wood is actually a veneered cladding on a structural fiberglass window. In hind-sight I should have gone with saeple or vertical grain douglas fir. At least the door is going to be QSWO.

[Edward Dyas]

I don't know that there is a difference between the finished product with pure tung oil vs polymerized tung oil. The drying time on polymerized tung oil is faster than pure tung oil is the main difference. It is a very slow drying finish and this time of year may take more than a week to dry. The best way to tell if tung oil is dry enough for another coat is to briskly rub the finish with a clean dry cloth and see if the tung oil smell comes off on the rag. When there is no smell it's ready.

If you are looking for the best spar varnish, that would be Epifanes. It's sold by places that sell boat supplies. It's a bit expensive but in this case you get what you pay for. A mid-range spar varnish I've had good luck with is Cabot Spar Varnish. The last I bought I got at Lowe's.

Well at least if the wood is just veneer you won't have problems with rot. Worst case scenario is having to replace a piece of veneer or paint over it. You can make red oak work but you have to keep an eye on it. When the finish starts looking dead and dry, scuff sand it and put another coat of varnish on before it dries out enough to let water penetrate. A lot of people think a finish is like a sheet of glass blocking out water 100% but it's not. A finish is porous like anything else and a certain amount of water goes through. It's just when the finish is fresh there isn't enough water going through to harm anything. As it ages though it gets more and more porous until enough water gets through to cause the finish to lift. By giving it a fresh coat from time to time it prevents the varnish from getting that porous.