Category:

Why Wood Weathers

4/26/2017

Even though pressure treated yellow pine, western red cedar and redwood lumber are extremely durable materials, their useful lives depend upon their environment. Left unprotected, these woods suffer photo degradation by ultraviolet light (sunlight); leaching (water absorption into wood components); hydrolysis (as in acid rain attack); shrinking and swelling caused by water absorption into and evaporation from the wood; and finally, discoloration and degradation by decay promoting micro-organisms.

Photo degradation By Sunlight
Solar radiation is the most damaging component of the outdoor environment affecting every surface. The decomposition of the wood is signified by the gray color that appears. This fiber loss occurs due to the sunlight destroying the lignin, the so called "wood fiber glue". Rainwater washes away these "loose" wood fibers and decomposed lignin exposing fresh wood to start the process all over. In addition, microscopic cracks and splits develop, allowing deeper water penetration. This mechanism is very evident in exposed structures located in the southern states.

Moisture Degradation
The sun and rain cycle causes moisture fluctuations in the wood resulting in shrinking and swelling stresses. Deeper checks and splits develop from these stresses causing all wood to cup, warp, curl, split and check at an accelerated rate. This process is accentuated in Douglas Fir, Pine, Spruce and Hemlock, which experience greater dimensional changes than cedar or redwood, with moisture loss and absorption. Some of these dimensionally unstable species are common pressure-treated candidates. Osmose, Woman, Borates and CCA are common pressure-treat systems or chemical names. Therefore, protecting the wood fiber of pressure-treated lumber from rapid moisture absorption (fiber swelling) and moisture loss (wood shrinking) is crucial to preventing premature wood failure modes not associated with rot. Hence, maintaining long term water repellency of the wood is an imperative coating property.

Wood Destroying Fungi
The natural decay resistance for western red cedar and redwood is due to their heartwood chemical components, including the thujaplicins and a variety of phenolic compounds. The thujaplicins contribute to the decay resistance of redwood and cedar. The phenolic compounds and resins give cedar its water repellency and lubricity (slippery surface). Both the phenolics and thujaplicins are water soluble and accordingly lost during use. As these compounds are lost, wood destroying fungi colonies may develop in the wood. This type of attack is characterized by the wood edges becoming soft and spongy, stringy, pitted, cracked and crumbly. Fallen leaves, continuously shaded areas, dirt accumulation, and constant contact with moisture (sprinklers and a heavy dew factor) can encourage fungi growth. This type of attack is characteristic of high moisture areas such as the Pacific Northwest and Southeast United States.

Older Trees are Better
Another important consideration is that new cedar and redwood lumber is being processed from younger, less decay resistant trees since the old, high natural preservative content logs are becoming harder to find. The life expectancy therefore is shortened. These factors, coupled with an increased awareness of various environmental issues in recent years to protect old growth timber, are further reducing the supply for high quality wood. This supply shortage represents a price increase of 50% to 150% on prices for cedar & redwood and it is affecting southern yellow pine also. This leads to the obvious conclusion that taking care of aged, older wood has become an economic necessity on your house.

The Like New Restoration Process For Wood
Fortunately, wood can be restored most cases to that "Like New" look. However, the transformation process is tedious involving repair, mold and mildew removal, wood surface preparation (removing the gray) and coating application. The toughest portion of the process, in terms of time and the cost, is the wood surface preparation procedures - including mold and mildew removal. The most important single decision in the process is which protective coating to use. If the right material is selected, that rich gorgeous restored look can be easily maintained for years - per the latest testing results (shorter in high traffic areas). If the wrong material is selected, the entire restoration process may need to be repeated in as little as 3-6 months to maintain that desired look.

Organic Growth Removal and High Pressure Water Washing
Mold and mildew are the dark gray or black growth we see on wood. All organic growth must be completely removed through surface stripping, high pressure water washing or a combination of the two. The restoration professionals generally use a combination of the two to more efficiently remove the top layer of weathered wood (gray wood) and organic growth at the same time. The process involves application of a diluted stripper (best) or special wood bleach) followed by high pressure water washing.

- Cautions -
Plant and vegetation protection is important, particularly when using bleach or oxalic acid. Oxalic acid's main industrial use is to etch metal. It has a strong odor and can cause repetitive coughing to those close to the applied material. USE A RESPIRATOR. Packaged properly with special surfactants and detergents, oxalic acid based products can be much safer and easier to use than bleach. High pressure water washers can be damaging to property and dangerous to the operator. BE CAREFUL!

Selecting a Natural Finish
Now that we have discussed how to get your deck or patio back to that "like new" look, we must consider how to maintain that "like new" look---preferably for a long time. The bad news is that most natural finish materials look great initially but last for only a few months as evidenced by wood "graying" or loss of water repellency. Most semi-transparent stains and varnishes (known as film formers) do not fair much better, with cracking and peeling being a major problem. It appears that the more highly advertised the product, the shorter it's life expectancy. Now for the good news. In this section, we will discuss the latest Texas Forest Products Lab (part of the Texas A&M University system) longevity testing and how to select the easiest systems to maintain. These results are taken from standardized roofing coating test which are the closest applicable standardized tests for the simulation of horizontal deck applications. We will elaborate on why penetrating oil-base systems are superior to water-borne systems, film formers and varnishes from the performance and "maintainability" standpoints.

The Toughest Weathering Tests
What makes the Texas Forestry's work so significant for those living in Southern and Southwestern states is that the natural weathering tests are conducted at a similar global latitude to ours. This closely duplicates the tortuous sun exposure we experience in Southern states, plus adds the damaging impact of the extremely high Gulf Coast rainfall and humidity. Those in the Northern States can feel good about Texas Forestry's work since the additional sun exposure makes testing more rigorous than could be achieved in the Northern States. The most meaningful testing that the Texas Forest Service provides is natural weathering. They have proven that accelerated weathered testing for natural wood finishes has no direct bearing on actual field performance longevity. Natural wood finishes that have been exposed to what normally is translated to a 3-5 year exposure (standard 1000 hour test for paints) failed in less than 2 years when subjected to natural weathering exposures. Therefore, when companies talk about accelerated weathering as their only basis for performance or long warranties (and usually high prices with warranty backing), run the other way! Natural south facing exposure-testing for sun resistance and north-facing exposure for organic growth resistance are the only meaningful proven tests. Some new accelerated tests utilizing mirrors or black box exposure to magnify the sun's effects are showing promise but I am not aware of any standardized correlation's yet for these types of exposures.

Oil Versus Water-Borne Finishes
For wood, in most southern states, the sun is Public Enemy #1. The sun is directly responsible for the intense drying effects leading to cupping, curling, cracking, splitting and surface checking. Water-borne treatments do little to alter these natural processes, but some are effective at controlling mold and mildew. Therefore, oil-borne treatments are recommended since they replenish wood oils that have been oxidized by the sun or washed out by the rain. Proper high flash point parraffinic oils (the new industry standard) are not only oxidation resistant, but also do not contribute to wood flammability (a consideration mainly for wood roofs). Though somewhat more expensive, oil-borne treatments are recommended over water-borne materials because of their superior performance. Wood is the pipeline for nutrients. The wood fiber is basically a series of straws that shrink and grow with moisture loss and absorption. By absorbing oil, these straws tend to regain part of their original size which reduces internal wood stresses as well as the volume available to be occupied by water upon exposure to rain or any other moisture source.

Natural Finishes vs. Semi-transparent; Transparent vs. Clear
Most people prefer a natural wood finish (full grain character allow to show though without a painted look) to full body stains. By natural finishes we are referring to clear or transparent finishes. Semi-transparent or full body stains are "thinned" paints (lower in pigment and resins solids) utilizing inexpensive, standard paint pigments (colored particles). These standard paint pigments have a large particle size which, at least partially, hides the grain from view. On the positive side, these large pigment particles provide valuable sun protection for the wood and coating itself. Be sure to understand the difference between "transparent" and "clear" coatings. "Clear" means absence of color. Transparent coatings have a special type pigment that will not block one's view of the wood grain from which the name transparent is derived. These transparent-oxide pigments (trans-oxides) are expensive but when utilized in natural finishes provide the desired; an obstructed full view of the wood couple with the coating itself. The very small particle size "trans-oxides" appear to our eyes almost like "dyes" on the wood surface when used in routine concentrations. Neither manufacturers or the buying public can afford to place enough UV absorbers and light stabilizers in a "clear" material to provide adequate sun protection by themselves. The sun protection provided by pigments are as important to the longevity of the coating system as they are to the protection of the wood surface. Be informed that the true "clear" materials will allow the wood gray or age quicker as well as degradation of the coating itself will be accelerated over that experienced by trans- oxide pigmented systems. Pigmentation helps extends the life of the coating system resins (binder that holds everything together) By physically blocking the damaging UV rays from contact with substantial portions of the vulnerable resins. The benefits of pigmentation appear to increase as the sun exposure increases (in other words, pigmentation is more important in the southern states than the northern states). We always recommend pigmented systems for exterior use to maximize wood and coating system longevity.

Maintenance Considerations:

Film-Formers vs. Penetrating Finishes
The problems associated with materials that form a surface film is that they tend to crack and peel when failure occurs. That means that chemical stripping or sandblasting is necessary to restore the substrate to a true natural look for reapplication. This situation is generally true of varnishes and most semi-transparent or full-body stains. The look of varnish is exceptional, but most of the varnish systems that I am aware of fail by cracking and peeling due to wood expanding and contracting. Once that starts, moisture seeps behind the film causing water staining and lifting of the coating off the wood and creating the ideal conditions for mildew or mold growth. The only viable solution to remove the water stains and unsightly black and gray growth is to chemically strip or sand blast/pressure wash the varnish (expensive). Then start over with an application of a fully penetrating, transparent natural wood finish. Some people have tried to minimize out of pocket costs by painting over the peeling material BIG MISTAKE!! Within a couple of years, the peeling film former and trapped organic growth shows up as discolored, peeling paint. Now the size of the problem is much larger and the cost of the "fix" is much greater.

Does Preservative "Mean Better"?
Some products are called preservatives. The word "preservative" carries the connotations that a material named should be the best, long lasting product available. Unfortunately, this simply is not true. The word preservative legally refers to an EPA registered component in the product which will affect fungus growth. If the claim is made that a product imparts fungus resistance into the wood structure itself, the complete product formula must be registered as a preservative. This "preservative" registration does not address water repellency, mildew resistance (different from fungus), UV resistance, longevity of the system or anything else of importance. The top-rated roof and deck system is not a registered preservative because it contains only a registered mildewcide and makes no claims about preservative properties. Yet by effectively controlling ultra-violet light and moisture loss and absorption for many years, it out performs most "registered preservatives" by a very wide margin. Therefore, be aware that the word "preservative" by itself does not mean much in today's world of high performance coatings. But, preservatives can be very important components of some systems---but it is only one component.

The Best Solution: Penetrating Oil-Based Finishes
To date, the best solution to the exterior wood finish dilemma is to use penetrating oil-based finishes that do not form surface films, and hence, do not pose serious lifting or peeling problems when the time to reapply is apparent. The resins (binders) for these systems set-up in the wood pores and do not form continuous films in classical sense. Maintenance then becomes as simple as cleaning the dirt from the surface, allowing the wood to dry and reapplying another coat.

Much cheaper, much easier, much faster than stripping or scrapping off old, peeling films. For restoration, the best of these systems contain a high percentage of free oil to restore the wood's moisture content as well as provide additional water repellency for the wood. The top systems in this category range in solids content from 60-97% (solids are defined as that portion of the system that will not evaporate upon use, or in other words, the important stuff that works). Generally, the higher the solid content the better. In contrast, many of the common name brand sealers on the market contain solids that range from 2-15% -very low. Besides being somewhat more expensive (for obvious reasons), The only drawbacks of these ultra-high performance systems are that they can take up to a few days to dry (particularly in cool weather) and soil somewhat more easy (routine rinsing and /or cleaning with dishwasher soap is a major help here). But the long-term protection and color maintenance is by far the best that is currently available for any type natural wood application.

The Natural Deck Coating Dilemma
The combination of foot traffic, pets, kids, sun, dirt build up and standing water (occurs every day if you have nightly dew) are very hard on deck coatings. Most of these natural finishes are designed to provide minimal blockage of the wood grain (essentially look as if they are not present) yet provide protection from everything. Long term color maintenance on decks is just not feasible with just one single application per deck. For roofs and sidewalls, 5 years of color maintenance in the southern states are capable of that life expectancy. Only a couple systems are capable of the life expectancy per the Texas Forest Service work.

The Expected Life of "The Best" for Decks
It is anticipated that the natural wood finishes with a high solids content when applied to fully exposed south or west facing decks will provided color maintenance for 2 to 3 years on aged wood in the southern states. These same systems generally provide longer term color maintenance as the sun exposure lessens or as one moves to a more northern climate. On new wood, a light coat should be applied to the wood to act as a stabilizer. This prevents the wood from splitting and checking due to moisture loss from drying and to get better penetration due to mill glazing. A reapplication of material and a light cleaning may be required after one year providing the wood with a penetrating finish.

Warranties, Do They Mean Anything?
I have seen people offering products for sale with warranties up to 30 years from purchase date. I have to laugh since we know there are only a few products that will last even 5 years on sidewalls and roof applications. These impossible warranties are offered by marketing people who believe they can sell anything - and they are usually right - until the market becomes wise to the misleading product performance. Normally the fine print says that 4 coats must be applied over 60 days and inspected by a company inspector or a power washing must be done every 6 months. One company offering a 25-year warranty required $1.20 to 1.50/sq. ft. be spent for coating material alone. The point of this discussion is to make you cautious when buying a natural wood finish and not fall for something that is too good to be true, because it probably is.

Important Points to Consider

Finish must contain resins (plastic binders) to minimize uneven color loss and streaking. This problem can be particularly true of some water-borne treatments. Pure oils are always subject to water wash-out or absorption by rain or dew unless protected by a resin barrier.
Make sure the product will "bead" water for several minutes - much like water beads on the hood of a freshly waxed car. The water repellency feature will minimize water penetration into the wood and reduce subsequent wood damage. When water hold-out disappears, it is time for another coat.
Paraffinic oils generally have a very low odor compared to their naphthenic counterparts. This can be important in high density apartment a condominium complexes or for persons with known sinus conditions, chronic allergies or other known health conditions. The napthenic oils (referred to as roof or floor oils) commonly used in inexpensive siding and roof treatments generally degrade in 9-18 months based on Forest Products Laboratory work. Paraffins will last 3-5 times longer under the same conditions.
Flammability test work (conducted by the Forest Products Lab on high flash point paraffinic oils) has shown that these materials have no effect on the flash point of the wood tested. This flammability work was comprehensive in that testing was conducted on fire retarded wood (cedar shakes), aged wood and freshly cut (green) wood with the same results.
Material must contain mildew and mold inhibitors, particularly in humid environments or in the presence of large amounts of water.
Linseed oil based products are not recommended for this particular microclimate. Linseed oil and other natural oils have some very good finish properties but contain glycerin and other sugars. To mold and mildew these act as a food source. Remember, natural oils are the lifeline of nutrients in trees or plants, and these plant nutrients will still support life - this is what we are trying to prevent from happening with your wood.
Pentachorophenol and mercury are strong enough to do the job but are presently unacceptable for the environment.

Recommended Products
Based on Texas Forest Products Lab testing, the following products are recommended for homeowners based on availability, price and excellent performance.