Why The Need To Protect Wood?
Although
wood is naturally durable and strong, the effects of outdoor exposure on wood
can destroy not only these physical properties, but its natural beauty as well.
Wood exposed outdoors is subjected to both rain (moisture) and sun (ultraviolet
radiation), two major factors that can ultimately cause it to lose its natural
beauty and strength.
Moisture
Water is one of wood's worst
enemies. Whether in the form of vapor or liquid, water can cause shrinking and
swelling, which can led to dimensional changes of the wood and degradation of
the finish. Water can also lead to decay or rot to occur in wood. In general, it
accelerates the weathering of wood exposed outdoors.
Shrinking
and Swelling
Wood shrinks as it loses moisture and swells as it gains moisture. More
precisely, wood only changes dimension between an absolutely dry state
(completely free of moisture) and its fiber saturation point (the point at which
the wood fibers are completely saturated with moisture). This fiber saturation
point typically occurs at about 30% moisture for most species. At this point,
all the wood's water is bound within the cell wall. As moisture content changes
above fiber saturation, the cell cavities take on or lose unbound water but the
wood cell walls do not change dimensionally. Below the fiber saturation point,
however, the wood will change dimension with changing moisture content. A large
percentage of wood finish degradation (e.g., painT/stain defects, peeling,
cracking results from moisture changes in the wood and subsequent dimensional
instability.
Water,
Water Vapor Effects
Shrinking and swelling of wood occur whether the water is in the form of vapor
or liquid. For example, wood swells during periods of high humidity and shrinks
during periods of low humidity; it also swells and shrinks as it gets wet from
rain and then dries. As discussed, wood can swell until it reaches fiber
saturation point. Rain water can cause wood to quickly reach, or even go beyond,
fiber saturation. Problems with poor performance of wood or finishes usually
occur when the moisture content of wood reaches or goes beyond fiber saturation.
The wetting/drying cycle of wood exposed outdoors can raise the grain of the
wood, resulting in a rough surface. On flat-grained lumber, the raised grain may
appear as thin knife-like feathers along the earlywood/latewood interface. This
degradation leads to eventual checking and cracking of the wood, increasing
water uptake, thus accelerating the degradation process.
Discolorations
Moisture contributes to many common wood discolorations--extractive bleeding,
iron stain, and mildew. These discolorations, although not harmful to the wood,
offer an objectionable appearance to the wood surface.
Extractive
Bleeding
Certain types of lumber such as cedar, redwood, mahogany, Douglas fir, etc.,
contain water soluble extractives (tannic acid) that can be leached to the
surface of the wood. The action of the sun can draw moisture (water vapor) from
inside the wood through to the surface. As this moisture is absorbed through the
wood, extractives are dissolved and then deposited on the exterior surface as
the water evaporates. The deposited tannic acid appears as a reddish brown
stain. Tannic acid in wood is what gives the wood its natural color and
resistance to rot and decay. Extractive-related discolorations can be difficult
to remove if they are allowed to remain on a surface for a long period of time
.
Mildew
Definition
Mildew
Mildew is an airborne fungus that can settle and grow on any surface that
provides a food source. Among the food sources are wood, plant and tree pollens,
bacteria, dirt, and other organic materials. The latter three are often found on
wood or on finished surfaces. Mildew spores are microscopic and, therefore,
difficult to detect until they colonize. They are transported easily by air
currents, insects, and animals. Due to its adaptability and myriad of species,
mildew is very hard to control and impossible to totally eliminate. Mildew fungi
appear in various colors but are commonly black. In order for mildew spores to
form colonies which are more visible, they need food, moisture, air (oxygen) and
ideal temperatures (usually between 40 and 90 degrees F.). It is next to
impossible to control these four requirements, therefore, paint and stain
companies incorporate mildewcides in their products to retard mildew growth. The
mildewcides found in paints and stains are not effective in destroying
pre-existing mildew. Therefore, it is very important that mildew be completely
destroyed prior to staining or painting. Of the four requirements needed for
mildew colonies to form, moisture is the most important and may be the easiest
to control. If wood is allowed to remain in a damp condition over an extended
period of time, mildew will spread. Because mildew has a great attraction to
organic materials as a food source, it has a greater tendency to grow on
surfaces such as wood, leather, and paper versus inorganic surfaces such as
aluminum, vinyl, and glass. Therefore, because of the mildew's stronger
attraction to organic materials, it will have a greater tendency to form and
develop on wood surfaces than on painted wood, particularly wood painted with
synthetic organic polymers such as acrylics.
The term "mildew" is a
common term in the paint and coating industry describing an unsightly
discoloration of paint film. Mildew is typically a superficial growth produced
by fungi. This growth can occur on a variety of surfaces both organic and
inorganic origins. Surfaces can be inanimate, such as wood, vinyl, and aluminium
substrates; or living materials such as plants. Mildew can also grow on
superficial surfaces such as dirt, grease and other industrial pollutants(1).
Moisture is the primary environment component necessary for mildew growth,
followed in lesser degree by temperature. Consequently, tropical areas have the
highest moisture (humidity) and temperature profiles provide the greatest
geographic challenges to mildew growth prevention. Hot, dry climates, as you
would suspect, see much less mildew growth. Fungal spores are present in air at
counts of 100 to over 1000 per m3 of air, depending on geographic location. (2)
Even freshly milled wood is immediately exposed to fungi regardless of location.
Approximately 100,000 species of fungi exist(3), many of which affect the
appearance and performance of coatings.
Effect on Appearance and
Performance
Mildew affects coatings in both
appearance and performance. Mildew generally appears in two forms, a spore type,
which resembles caviar in appearance, or mycelium or filament type. Mildew
generally appears as an unsightly discoloration on a coating, thereby making the
appearance unacceptable. The performance of a coating may be compromised either
by mildew growth on the coating, or by the application of the coating to a
mildewed surface. When mildew grows on the coating, increased dirt pickup,
cracking, flaking, and loss of adhesion can be realized. When a sound coating is
applied to a mildewed surface, the adhesion of that coating to the substrate
will be reduced due to the physical interference of the mildew. When an infected
coating is applied to a substrate, the above failures can occur even more
rapidly.
Interference with Adhesion
Paints are designed to have
excellent adhesion to a variety of surfaces. Depending on the paint
manufacturer's intentions, a particular paint may be designed to adhere to wood,
masonry, vinyl, aluminium and/or other substrates. The presence of foreign
matters such as, oil, grease, dirt, mildew, tree pollens and/or other substances
on a substrate interferes with the adhesion characteristics of the paint.(4) It
is in the homeowner's best interest to ensure that the substrate to be painted
is properly prepared for painting.
Intercoat Adhesion of Paints
When mildew has grown on a painted
surface, a home owner may be inclined to improve the appearance by applying a
coat of paint directly over the mildewed surface. The approach is not a
permanent cure. Because of the infestation below that new paint surface, mildew
will appear again on the new surface.(5) In addition to the poor appearance of
the mildew, a greater problem now exists with the intercoat adhesion of the new
paint film on the old paint film. By not following a good surface preparation
procedure, the homeowner has allowed the mildew to exist between the old and new
coats of paint. Intuitively and quantitatively, we know that this mildew
interferes with the new coating's ability to adhere to the old coating. Early
failures such as cracking, flaking, blistering, and more can be expected. To
ensure the best intercoat adhesion, an acceptable method of surface preparation
must be followed.
Effect of mildew on Wood
We have discussed the harmful
effects that mildew has on the paint film. However, mildew can also cause
serious damage to the wood substrate. The cellulose and resins found in all wood
species are nutrients for mildew. In essence, the wood substrate serves as a
food source for mildew. If the surface has not been cleaned and prepared
properly, or if the paint itself has little or no inherent mildew resistance
(more later), the wood substrate will be damaged by the mildew growth. As this
damage continues, the substrate's integrity will become compromised and eventual
failure of the paint is unavoidable.(6)
Necessary Contributing Factors
regardless of specific species,
all fungi require oxygen, water, a food source, and some narrow temperature
range to metabolize and reproduce. Typically, fungi cause problems with dry film
only, and do not effect the coating in the can as there is usually insufficient
oxygen present for metabolism. Fungi need water to grow. Hyphae, small
threadlike structures, grow from the fungi into the coating and wood substrate,
essentially opening the substrate and making it more porous, thus prone to
moisture. Consequently, as the moisture content continues to rise, the fungi
flourish. For food, fungi generally metabolize starches, sugars, proteins, fats,
and cellulose. Temperatures from 70 and 90 degrees Fahrenheit are ideal for
fungi growth. Below freezing, fungi become dormant.
Other factors that can also
contribute to mildew growth include the type of coating and its surface
characteristics. Generally, top quality latex paints offer the best protection
from mildew. As the quality of the paint decreases, the chance for mildew growth
typically increases (a further discussion will follow under MILDEW CONTROL).
Also., latex paints generally show less mildew that alkyd paints.
Mildew Control
Surface Preparation
If mildew is already present on a substrate, the mildew but be killed and
removed before the substrate is finished, or else the mildew will grow through
the new coating. To kill mildew and remove mildew from the surface, follow the
steps outlined below:
1. Using a spray canister(one designed for insecticide
application will do) available at most hardware stores, apply the following
solution liberally to the substrate and allow to set for about 10-15 minutes:
·
a. 3 quarts water
·
b. One quart common household bleach
·
c. 1/4 cup maximum of liquid dishwasher detergent (ammonia-free)
The bleach is the key component that actually kills the mildew. Allowing this
solution to set for five minutes give the bleach time to settle into any
crevices and hard to reach place to kill all the mildew present. Skipping this
10-15 minute set time may result in an inadequate job. The detergent is added in
a small amount to help emulsify any mildew or dirt to aid in its removal. Liquid
dishwasher detergent is the best choice it will not foam like regular dish or
laundry detergent.
2. Using the same solution, wash the substrate clean
using a power washer. A Second choice if a power washer is unavailable is to
scrub the surface with the same solution. Far masonry substrates use a wire
brush. For wood, use a softer bristle brush. And for substrates sensitive to
abrasive damage like aluminium and vinyl siding, use a sponge.
If mildew was present on the original substrate or previous coats, and a new
finish coat is already applied, the mildew will grow through the coating. It is
usually impossible to stop mildew growth at this point. All the coatings must be
stripped down to the original substrate and then cleaned as described above
before applied a new coat. (7)
Mildewcides
Mildewcides are chemicals added to coating formulations to help stop mildew
growth on the coating. There are a wide variety of mildewcides used in the paint
and coatings industry.
Zinc Compounds
Zinc Oxide
Zinc Omadine
Zinc Borate
These chemicals are usually listed on the label of a coating product,
although exact amounts are not usually revealed. One particular combination of
mildewcides, Zinc Oxide and 2-n-octyl-4-isothiazolin-3-one, has shown a
synergistic effect when used in combination.(8) For the consumer, attempting to
study can label analysis to determine the best mildew resistance of a coating
would be tedious and non-productive. Simply the best way for the consumer to
gauge mildew performance is by the overall quality of the coating. Top quality
latex coatings will offer the best mildew protection in nearly all cases.
Pretreatments and Preservatives
Above we discussed surface preparation before recoating a substrate. After
proper surface preparation has been done, one may chose to apply a special
coating before applying the finishing coats. Types of special coatings include
primers, wood preservatives, and sometimes semi-transparent stains. Primers are
used when the finish coats are opaque. Paints, Opaque Stains and Timber Finishes
are examples of opaque finishes. Wood preservatives can be used under opaque
finishes, but are also very popular under semi-transparent stains.
Semi-transparent stains themselves can be considered a pretreatment when used
under a clear coating. This is popular in cases where someone wants the natural
look of a semi-transparent stain, but desires the added protection of a clear
coat.(9) Each type of pretreatment has its own recommendations for application
so following the label instructions is a necessity. Regardless of the type of
pretreatment, proper surface preparation is essential to insure adequate
performance.
Ultraviolet
Radiation (UV)
Regardless of the care taken in
building a structure, wood ages when exposed outdoors. The sun's ultraviolet
rays begin to degrade the surface of unprotected wood within weeks of outdoor
exposure. The sun's ultraviolet rays will change the color of the wood. Cedar
and Redwood will tend to get lighter after initial installation as the colored
extractives and lignin are removed. Light wood, such as pine and fir, tend to
get darker. Rain washes the degraded wood materials from the surface. Erosion is
more rapid in the less dense earlywood than in the latewood, which leads to an
uneven surface. In many cases, the wood erosion will also provide an uneven and
eventual darkening of the wood's surface as mildew develops. In some climates,
such as along the seashore, wood tends to weather to a silvery gray. This change
in color is followed by a loosening of wood fibers and gradual erosion of the
wood surface. The sun's ultraviolet rays also provide problems for clear natural
finishes.
Clear
Natural Finishes
Given this delicate balancing act,
stain and paint manufacturers have produced a number of clear products to meet
the aesthetic desires of the homeowner, as well as the protective requirements
of the wood. Clear natural finishes generally fit into three categories:
Water-Proofing/Water-Repellent Sealers, Wood Preservatives, and UV-Resistant
Clears (blocking or absorbing). Some clear natural finishes on the market today
may combine characteristics from more than one of these three categories. Air
quality regulations along with changing technology, have resulted in an
abundance of new clear formulas on the market. High solids, water-based, oil
emulsion, and traditional solvent-based are just a few examples of the different
types of clears sold within each of the three categories of clear natural
finishes. Also, some of these new clear natural finishes may be film forming and
not penetrate the wood's surface and are themselves subjected to the same
problems they are trying to prevent (moisture, UV degradation). Thus, when
choosing a clear product, it is important to match the aesthetic desires with
the protective requirements of the wood, and at the same time to understand the
performance limitations of the product.
Water-Proofing/Water-Repellent
Sealers
Water-proofing clear products are often confused with water-repellent
products. Water-proofing products form a membrane designed to withstand
hydrostatic pressure (like wind-driven rain) and keep water out of a home. Water
repellents shed water, like rain water or water from a sprinkler. Water-proofing
coatings are often elastomeric and are designed for brick, stucco and other
masonry surfaces, not wood. Clear products that fall into the water repellent
category are clear, penetrating finishes. They typically contain a large amount
of wax, usually paraffin, that decreases the amount of water absorbed into the
wood, thus preventing swelling, shrinking, warping, and splitting. Because
moisture is repelled, mildew growth is slowed. These products often provide
spectacular beading of water upon initial application. However, they offer wood
little protection from the sun's ultraviolet ray, resulting in the eventual
discoloration to a gray, and then black from the ultimate growth of mildew. In
addition, water repellent sealers rarely contain more than 20% of resin. When
water repellent sealers are subjected to degradation by the sun's ultraviolet
rays, the result is a loss in the water beading effect. The life expectancy of
these products typically ranges from six months to one year on horizontal
surfaces (decks), and on to two years on vertical surfaces.
Wood
Preserving Finishes
Wood-Preserving Clear Finishes differ from water repellent finishes in that
they also contain a minimum amount of an EPA approved and registered pesticide.
These clear finishes are very similar in appearance to
water-proofing/water-repellent sealers. If the pesticide is there to preserve
the wood, it's usually registered. If the pesticide is there to protect the
clear finish itself, it may or may not be registered. This is not to say that
pesticides (mildewcides and fungicides) are not important. Mildewcides and
fungicides are important to all clear natural finish formulations and most
manufacturers incorporate some mildewcides and/or fungicides into their
products. The addition of these specific EPA approved pesticides to the product
in specific quantities, and the subsequent EPA registrations, allows
manufacturers to use the words "Wood Preserving" when describing the
product. Some common pesticides/mildewcides used in these products which provide
decay resistance to wood include: 3-iodo-2-propynyl butyl carbamate, zinc
naphthenate, copper naphthenate, bis (tributyltin) oxide (TBTO) with N-tricholoromethylthio
phthalimide and copper-8-hydroxyquinolate.
Wood Preserving Clears contain a
preservative and wax, as well as a resin and solvent. The wax reduces the
absorption of liquid water by the wood, and the preservative prevents wood from
darkening by inhibiting the growth of mildew and decay organisms. However,
without special ultraviolet radiation absorbers, Wood Preserving Clear finishes
will not protect the wood surface from the sun's ultraviolet rays. The resulting
wood degradation causes the release of wood fibers, exposing underlying,
unprotected wood fibers, which turn gray and then black from fungi attack. In
addition, wood preserving finishes generally contain less than 20% binder or
resin. As with water proofing/water repellents, the wax in the product quickly
breaks down on the lateral surfaces and the water beading properties vanish. The
life expectancy of these products can range from six months to one year on
exposed surfaces. However, the water repellence absorbed in the end grain will
be effective for many years. Also, after the wood has gradually weathered,
additional treatments may last longer because the weathered boards absorb more
finish.
Paintable Wood Preserving Clears
may also be used as a treatment for bare wood before priming and painting or in
areas where old paint has peeled, exposing bare wood. This treatment keeps rain
or dew from penetrating the wood, especially at joints and on end grain, thus
decreasing the shrinking and swelling of the wood. As a result, less stress is
placed on the paint film, and its service life is extended. For treating bare
wood, make certain that the manufacturer's label indicates that the Wood
Preservative Clear is paintable. Some products have too much wax or other water
repellents and the paint will not adhere adequately.
Wood Preserving Clears are
becoming scarce in today's marketplace due to increased restrictions on the use
of some of the more commonly used pesticides. Also, the value of adding these
pesticides (cost versus long term durability of the finish), has caused
manufacturers to take a closer look at formulations containing the levels of
pesticides necessary to claim "Wood Preserving" properties. More
recently, the widespread availability and use of pressure-treated wood has also
had a negative affect on the popularity of Wood Preserving Clears.
UV
Resistant Clears
This is the broadest category of clear natural finishes on the market.
Pigments and organic chemical additives are used as light absorbers (or
screeners) to protect the products binder from degradation by the suns
ultraviolet rays. The pigments that are used most frequently are either a
transparent form of iron oxide that reflects UV radiation or ultrafine titanium
dioxide. The organic chemical additives are of two types--ultra violet absorbers
(U-VA) and hindered amine light stabilizers (HALS).
Ultraviolet absorbers are designed
to work by absorbing the sunlight, especially ultraviolet radiation and
dissipating the energy as heat. During this process, some are destroyed,
limiting the effective life of the product. Hindered amine light stabilizers (HALS)
act as scavengers for the components of the coating (binder and pigments) during
degradation, and through this process, block further degradation of the clear
finish and the wood. Generally, U-VA's, HALS, and pigments are used
synergistically to offer the best protection of the wood surface.
UV-Resistant Clears are, many
people feel, the solution to the delicate balancing act of the aesthetic desires
of the homeowner and the protective requirements for the wood. UV-Resistant
Clears are not "water" clear, but rather products that impart a slight
color tone from the use of transparent transoxide pigments or titanium dioxide
and penetrate into the wood surface. Thus with UV-Resistant Clears, various
"colors" are available. Transoxide pigments are translucent and thus
highlight the natural grain, texture, and beauty of the wood. Transoxide
pigments are also versatile, finding success not only in traditional solvent and
oil-based product formulas, but also in the latest formulas that meet tighter
air pollution (VOC) regulations, such as oil emulsion, high solid oils, and
water-based formulations. The life expectancy of a UV-Resistant Clear finish is
twice that of water-proofing/water-repellents and wood-preserving finishes and
can range from one to two years on horizontal surfaces (decks), to two to four
years on vertical surfaces.
