Restraining an exterior wood entry door. Any woodworkers in the house?

Mr_Asa said:

In reply to Curtis73 (Forum Supporter) :

In GRM terms, Dad talking woodworking is probably most equivalent to the Nelsons talking drag racing.

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Your dad can't break into the 8's either?
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Asas_Dad said:

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

If this isn't a "mic drop" moment, then nothing is! Wow, THANK-YOU for the taking the time not just to type that, but to share what is obviously a wealth of experience in the field(and I assume many others)! 

Mic drop

My thanks for the compliments. I forgot to include a disclaimer. I am not a coatings scientist, but I am a graduate of the Smithsonian’s Furniture Conservation Training course, and I have going on 40 years of experience with furniture and finishing. Between the two I learned something about finish chemistry, mostly antique coatings, but some of modern ones, too.

Although the coatings industry is getting way ahead of me...

That was very informative.  The only thing I would want to add about my own quote, and I appreciate you directly referencing my previous addition to this thread, is that there are products advertised as "water based polyurethane".  What exactly is in the can, I don't know, and I don't know the exact chemical makeup of oil-based poly's.  I was simply suggesting not using the water based stuff for this application.

I think there's a whole lot of products that are hard to know their true makeup.  It leads to this situation where people suggest brand names of products that they have experiences with.

What's the supposed correct collection of products to use here?  Also, though MEC-based strippers are gone from store shelves for all but a few people, aren't strippers still somewhat useful in removing most finishes (they aren't as good now, not debating that at all).

Guys: I couldn’t let this alone, had to keep picking at it, went looking for UV stabilizers for polyurethane varnish. I found this, it is an excellent overview of the varnishes, and I recommend reading it, it’s only 3 pages. https://www.chemical-supermarket.com/home.php?printable=Y&cat=128  Notice how in the first section on Water-Resistant Compounds it states “we can eliminate current technology epoxides as topcoats due to their lack of UV resistance and their incompatibility with UV absorbing additives.  We can also eliminate conventional varnishes and paints due to their incompatibility with antioxidants.” Good to know.

The stabilizers can be bought as an additive, but I haven’t gone looking for them. The trade name for one that includes both HALS and UVA is Tinuvin, made by BASF; but there are hundreds of different formulations of Tinuvin, so you’ll need to research which is compatible with your product.  They see wide use in all sorts of plastics.  Next, my sense is that very little of it is required, so you’ll want to get mix ratios as well. Finally, I have no idea of cost. Who knows if a one-ounce bottle of which you will use one eyedropper per quart will cost $100? If someone goes deeper into this, please let the rest of us know.

Lastly, to reiterate, this stuff only extends the life of a coating; it doesn’t make it last forever.

Lotusseven7, we all have our strengths, this is mine. I would never undertake the builds I’ve seen on GRM.  I have always worked on my own vehicles, with moderate success, but I’m really not that good at, nor do I enjoy it - or the skinned knuckles. I’ve always said that I’m a mechanic only in self-defense.

Pres589, I can’t recommend one product over another, because a) you never know what’s in it, and b) they change all the time, even within a brand. If I need a polyurethane for some project I just grab what’s on the shelf at Lowe’s or HD, but aware of the limitations it will have. For automotive clear, go to your auto paint supplier; I’ve been playing with it, it sprays more like lacquer. As for strippers, yes, I’ve used them my entire career; the first I ever used actually had benzene, a known carcinogen, in it.  Worked great; I would never even open a can of it now.  None of the strippers being sold now work worth a damn.