Question
We built a mahogany door for a client. The door gets hammered by the sun all day. As a result, the finish has faded and deteriorated quickly. Are there any suggestions as to what stain and what finish to use on the exterior side of the door that will essentially be bulletproof?
Forum Responses
(Finishing Forum)
White paint is the ultimate in durability. Higher concentrations of pigment improve the UV resistance, as do lighter colors. So white paint is the extreme end of that continuum. Transparent reds, greens, and clears represent the other end of the continuum. Blacks and dark browns seem to have good durability despite absorbing most of the UV rays. These very dark colors have some other drawbacks, though. They tend to get hot in summer sunlight. Metal doors frequently experience severe expansion problems when painted in these dark tones. The finish that you are seeking is not in existence. You'll have to settle for one of the less than perfect options that we do have. There is a post here about twice a month looking for a clear or transparent stain finish for exterior doors that will not suffer swift weathering problems... such a boon is yet to be discovered.
Now for stain, use only pigmented wiping stains. Beware - some lines of stains don't tell you which colors are dye or pigment. If it doesn't have a considerable amount of sediment in the bottom of the can, it could just be a dye stain which will fade rapidly.
For application in a typical shop environment, Chemcraft's D-Dur seems to rule the roost here in the pacific northwest. It is an old Sadolin formula plural component 2k urethane. It is microporous, allowing moisture trapped under the coating to escape without popping the coating. We have doors with southern exposure, no protection, going strong after 5 years. Some of these doors have had glaze and toner effects incorporated in to the finishing procedure and still look great.
Sikkens Cetol 1 & Cetol TGL have great on-the-jobsite application properties. Primarily, these are brushing products (have had limited success spraying). Again, Sikkens offers a microporus coating. Although the Sikkens product seems to need maintenance coating in southern Exposure in 2 to 3 years, it holds up better than spar varnish or typical single component exterior clear coating.
I'm a paint chemist, formulate my own products and I've been doing this for over thirty years. You can find my main company website through my Five Year Clear website previously posted. I'd like to make a few general comments about why clear coatings fail, even good ones, and actions that can extend their lives.
I have bought very decent varnish such as Epifanes with lots of Ultraviolet Absorber, and I find it does not say on the can how much to use over a given area. Any film-forming finish that contains ultraviolet absorbers [UVA] needs to be applied to a controlled film thickness, in order that the UVA be able to effectively protect the underlying wood. A UVA in some concentration will attenuate a certain percentage of the UV for each unit thickness. For example, one percent UVA in a varnish might attenuate ten percent per mil of film thickness (I am making up numbers, but they aren't that far off) . That means if you have a five mil film, assume you get fifty percent attenuation, and a second five mils gives fifty percent attenuation of what got through the first five mils, or 25% of the original, and another five mils attenuates that by half, so we now have about twelve percent of what ultraviolet was in the original sunlight. Eventually the ultraviolet is attenuated enough that the small amount of what gets through to the wood does not do enough damage before other failure mechanisms cause failure for other reasons.
Dry film thickness determines life.
Other factors affect it also. Life may be the wood losing its color, or the film losing its gloss, or the film losing its adhesion. Those are the three main failure mechanisms. Each may have more than one cause. I will try to cover a little bit of this area with less than five thousand words.
When a manufacturer of a clear finish does not give a coverage requirement, they are omitting vital data which will affect the quality of your work, and that will reflect badly on their product as well as you. If it does not say in the literature of whatever clear finish you are using, you should contact the manufacturer and ask them, for their particular product, what the relationship is between coverage and maintenance of the color of the underlying wood, and if some recoating schedule (“maintenance coats”) is required for color maintenance.
Gloss maintenance is another issue, and this depends on the resin technology (varnish versus urethane-modified varnishes versus two-component polyurethanes) as well as what sort of magic (I’m a paint chemist. We have our secrets.) is in the formulation to give better gloss maintenance. Even the best varnishes will lose their gloss in six to twelve months, and it has to do with the resin technology. The two-component polyurethanes can be the best, depending on the formulation technology. I personally think mine is the best out there (On boats, my customers tell me it looks like the day it was done even five years later, and that the gloss goes between the eighth and ninth years in Dan Diego, CA or Ensenada, Mexico.) but I warned you at the beginning I was prejudiced.
Many clear finishes are low-sheen, and gloss maintenance is not an issue, and even a good varnish will do well on a front door if applied to an adequate film thickness, fully cured, and wet-sanded with soapy water and an abrasive pad to give the matte finish. I would budget ten to fifteen square feet per quart of any varnish with what they advertise as a high level of ultraviolet absorbers for this application. Varnish, by the way, means a resin made from the drying oils of certain plants. “Conversion varnishes” (and other corrupt names for coatings) are not real varnish. Many coating manufacturers make up names to impress, and trying to understand what things are really made of can become confusing.
Polyurethane-modified finishes, whether one- or two-part, will have better abrasion resistance than a conventional varnish, and even though a conventional varnish is the easiest to touch up (if a matte finish), the customer may need the higher-mechanical-performance of a more sophisticated finish.
Another reason for failure of the whole spectrum of clear finishes on wood is the chemical interface with the wood itself. I am not a big fan of waterborne finishes on wood for the reason that the first thing the water does is hydrate the cellulose fibers right at the surface, and this interferes with the adhesion of the resin phase to those damp fibers. I do not care for two-component polyurethane technologies applied directly to wood (as some other manufacturers recommend) for a different technical reason, namely that wood naturally contains moisture hydrated onto those wood fibers, and water destroys the isocyanate resins that are the curing agent in those products, including mine. That entire class of chemicals is very moisture-sensitive, and the cans contain various warnings about it. I don’t think a proper chemical cure and reliable chemical adhesion will develop. I came up with a different approach to the general subject of creating coating adhesion to wood over thirty years ago, and I called it Clear Penetrating Epoxy Sealer.
Back when I first got into this business, some of my paint customers asked me for better wood-finish technology. I reasoned that in order to penetrate wood with its natural moisture level, and sometimes excessive moisture, a solvent system that could dissolve water was needed. I further thought that the most wood-compatible sort of chemically-curing resin system would be something made largely from the natural resins of wood itself, and I used an epoxy-curing-mechanism when I formulated that product as they are versatile and relatively insensitive to air humidity. It proved very successful, for additional reasons I only found out about and fully understood twenty years later.
Water vapor passes in and out of wood. Thirty years ago I did not realize how important it was for the wood to be able to “breathe” in the region directly under a coating. I learned this over the next decades form my painting contractor customers. Coatings, particularly paint on buildings, fail because the water vapor accumulates under the paint film when there is dew on the outside, and when the day warm up the fungal spores hatch and begin to eat away the wood fiber under the paint film. It turns out that the Clear Penetrating Epoxy Sealer allows water vapor to pass, but not liquid water, and this capability plus the gluing of the surface fibers of wood down into the bulk of the wood, gave a substrate that resisted deterioration and promoted much better adhesion of any paint, varnish or other coating than anything else manufactured.
Comment from contributor J:
I am a residential painting contractor who specializes in finishing and refinshing entry doors and related exterior wood products. My search for a product that will hold up excellently under severe weather conditions (hot sun, moisture, etc.) has led me to purchase a catalytic urethane made by Sherwin Williams. It is not available at all stores, but must be ordered from their chemical mixing facilities. The system is a three part (two if you do not use the retardant). It is a very expensive product, and is designed to be sprayed. For information or specific details, contact them at their San Diego, California facility.
