Clear Finishes on Exterior Wood:
This question raises its head on a regular basis, by homeowners, wooden boat owners, anyone who wants to show off the grain in a pretty piece of wood that lives outdoors. There’s not a good answer.
In the photos I see three distinct problems: fading of the stain, bleaching of the wood, degradation of the previous varnish. All are light-induced; the higher the energy of the light (UV), the faster, more severe the damage. I have personally seen an unpainted south-facing wooden window frame inside an 18th century house in Colonial Williamsburg that was so continuously blasted by the sun through the glass that the wood itself was eroded, looked like driftwood.
My son, Mr Asa, is correct; wood continues to expand and contract in response to humidity cycling, if not forever, at least until the cells of it are degraded to non-structural. Wooden artifacts found in Pharaohs' tombs, 4,000 years old, have been shown to change dimension due to humidity, by even as little as the moisture breathed out by visitors in the tombs. The only way to prevent it is to enclose the wood in a sealed case. A finish coating won’t do that; all are to some extent permeable to moisture. Forest Products Laboratory tested numerous coatings for wood, found that the one that came the closest to 100% impermeability was two coats of dipped molten paraffin. Not a look you want for your door.
Most wood stains have very little actual pigment (finely-ground earth minerals) in them. The minerals are that which will resist fading. Dyes (color that penetrates the cells of the wood) fade even faster. The varnish will go simultaneously; the sun blasts apart the molecules that give it integrity. That can be seen in the third photo lotusseven7 sent; see the lifting of the varnish atop the molding over the lock rail? https://en.wikipedia.org/wiki/Photo-oxidation_of_polymers#Prevention The natural color of the wood will fade very quickly when any protection is gone.
As for drying oils, any that is pressed from the seed or flesh of a plant will eventually cross-link as it dries, giving a hard(ish) coating. Then it will continue to oxidize as it ages, will eventually fail. The only difference between tung, walnut, and linseed is how quickly it happens. Linseed goes the fastest. Who among us has not seen a walnut gunstock that has been lovingly oiled with linseed every year, that has turned black and gummy? And get this: the same FPL study of wood coatings showed that degraded linseed oil actually became hygroscopic, meaning it absorbed moisture.
pres589 (djronnebaum) mentioned oil-based polyurethane. Most people don’t understand that ALL alkyd urethanes are oil-based. The difference between them and traditional, older oil varnishes is that alkyd resin has replaced what used to be a natural (plant) resins. https://en.wikipedia.org/wiki/Alkyd
The greatest problem I see with asking such advice is that the recommendations you get are only for product names. The people making them may have some experience with that product, but no knowledge of what is in it, much less its aging characteristics. For example, Thompson’s Water Seal used to be beeswax dissolved in a hydrocarbon; I have no idea what’s in it now. This is not a criticism; it’s very difficult to find the ingredients of a varnish. I have done a few entry doors myself, in most cases my information was limited to the manufacturer’s claims.
So we’re wanting a finish coating that moves in concert with the wood, doesn’t crack from the movement or from age, blocks ultraviolet to prevent fading and wood damage. The short answer? Such a finish doesn’t exist. At least not yet.
Lotusseven7, you have the second-best orientation of your front door. Best would be north-facing, no direct sun. But even indirect sunlight will eventually break down a finish.
The best exterior finish I have ever seen was on a boat at the Antique Outboard meet with mahogany bow deck and seating furniture. I asked the owner what it was; he said it was four coats of a polyurethane (that he named, the brand name meant nothing to me) to build thickness, wet-sanded level and sprayed with two coats of automotive clear topcoat. (But after meeting him, I’m sure that boat never spent even one night outdoors, uncovered.)
The key is the automotive topcoat. For those of you that don’t already know, car paint color is no longer an enamel, it’s a seriously underbound color pigment followed by two coats of clear. The clear is a urethane, but it’s catalyzed (usually with an isocyanate hardener – wear a respirator!), and incorporates ultraviolet stabilizers to slow its breakdown. We’ve all seen older cars that the clear coat has failed; the underlying color is very quickly lost. Those ultraviolet inhibitors are Hindered Amine Light Stabilizers (HALS) and UV Absorbers (UVA). They, too have a limited lifespan, but it’s a lot longer than whatever is in most spar varnishes. HALS: https://en.wikipedia.org/wiki/Hindered_amine_light_stabilizers UVA: https://en.wikipedia.org/wiki/Polymer_stabilizers
How you strip or sand the wood will have no effect on the longevity of the finish, only how smooth it is. Solvent stripping is now out, however. The EPA or OSHA has banned methylene chloride in strippers to anyone not licensed to use it.
Another suggestion: install a glass storm door over the wooden one. Even “cheap” (soda) glass will block ultraviolet. It’s why you can’t get a tan from the sun coming through your windshield. You can even add a UV-blocking film to it if you wish.
-Dad
