For a long-lasting beautiful wood door finish, three things are required—the proper finish material, the proper application technique, and the proper surface preparation initially. Keep in mind that any clear coating is very thin when dry, so wood surface imperfections will show through.
The basic clear finishing material options for wood doors are urethane, varnish, and shellac. Urethane and varnish harden by a curing process. This is an actual change in the chemical structure when they are exposed to air or moisture. Shellac hardens by a drying process, and therefore, it is not as durable.
If you see the words “cross linking” on a can of clear door finish, it is probably durable. Cross linking means that the chemicals bond together into large chains of molecules as they cure. Think of it as how thread is woven into a strong cloth material. These finishes also block moisture very well.
Urethanes, at least the better-quality ones, are more durable than standard varnish. Urethanes use a special chemical to create extra-strong chains when cured. This makes it more scuff- and chemical-resistant. Urethane tends to yellow more slowly over the years than varnish does, but it does yellow some.
Make sure that you use door urethane and not floor finishing urethane. Door urethane contains ultraviolet (UV) light stabilizers to protect it and the door from the sun’s rays. UV rays can actually destroy the top surface of the wood under the finish.
You may consider having the urethane tinted for a unique look and to help protect the wood from the direct UV rays. The color pigments, even when only slightly tinted, in combination with the UV inhibitors will block more of the sun’s damaging rays.
Door urethane is also more flexible than floor urethane. It must be able to handle the door’s thermal expansions and contractions, from a hot summer afternoon to a cold winter night. Floor urethane is usually less able to withstand foot traffic and does not have to expand and contract much.
Surface preparation is key to a beautiful door no matter what finish you use. It must be clean and dust- and wax-free. Urethane has excellent adhesion, so it will stick to the dust and wax on a dirty door. If the door is not clean, within less than a year, the urethane will start to peel.
If you want a very high gloss final appearance, carefully sand the door initially. You must also lightly sand it between coats. For this type of finish, avoid water-based urethane that may lift the grain slightly. Do not use steel wool between coats, especially if you are using water-based urethane. The tiny residual steel particles can rust from moisture.
Read the manufacturer’s instructions carefully to find the length of recommended time between coats. Follow its recommended times precisely. Also, never shake urethane like you would paint. Stir it to mix it. Shaking urethane introduces fine air bubbles into it, and you can see them in the cured final surface.
