John Leeke's Historic HomeWorks™
207 773-2306 26 Higgins St. Portland, ME 04103
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John Leeke's Historic HomeWorks™ |
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207 773-2306 26 Higgins St. Portland, ME 04103 |
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| [Home][Library][Restoration Reports][Seminars][Forum][Internships][Office][Workshop][Front Porch][Search] info.© 1994-2009 JohnC.Leeke |
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"Finishing Wood
Decks"
Joint Coatings/Forest Products Committee
Chairman: Dr. Alan Ross, Kop-Coat, Inc., Pittsburgh, PA
(1-412-826-3387)
Abstract..........(Return to
Table of Contents)
Wood decks have become an important part of residential
construction in recent years. However, there is considerable
confusion regarding how these structures should be protected with
finish. This paper summarizes the types, application techniques,
and expected service lives of various finishes on both
preservative treated and untreated lumber. Recommendations are
made on the basis of decades of research on various wood species
using a wide variety of finishes.
Introduction..........(Return to
Table of Contents)
For optimal long-term performance, a wood deck should be
maintained with a finish after construction. The horizontal
surface, foot traffic, pooling of water, and full exposure to sun
and rain make deck finishing more demanding than other wood
finishing (e.g., exterior walls and interior floors). A full
range of both penetrating and film-forming finishes are
available, but the penetrating finishes provide better overall
performance and provide easier refinishing (Cassens and Feist,
1991).
The primary function of any wood finish is to protect the wood
surface from natural weathering processes (sunlight and water),
and to help maintain its appearance. How well the finish
accomplishes this function depends on how the finish was
formulated, particularly the type and amount of polymer (resin or
binder), pigment (including extenders), solvent system, and
additives used. Of these components, the type and amount of
polymer and pigment most directly affect the mechanical
properties and durability of the finish. Other factors that
affect the durability of a finish include application technique,
time between refinishing, extent to which the surface is
sheltered form the weather, and local climate conditions.
Available Finishes..........(Return to
Table of Contents)
Penetrating Finishes
The advantage of a penetrating finish over one that forms a film
is that the wood can breathe, and the finish cannot flake, crack,
or peel. Such treatments reduce water absorption, and retard the
growth of decay and staining microorganisms. Penetrating finishes
can be used as a pretreatment for other finishes or as a natural
finish for wood. There are three types of penetrating finish:
Water repellents, water-repellent preservatives, and
solvent-borne semi-transparent stains.
Water Repellent and Water-Repellent Preservatives
The difference between water repellent (WR) and a water-repellent
preservative (WRP) is the inclusion of a mildewcide in the WRP.
Mildewcides inhibit mildew growth on the surface, and provide
moderate protection against decay in aboveground use. The WRPs
are not intended for use in wood that is in contact with the
ground and should not be confused with the preservatives used to
pressure treat wood. The WRPs provide short-term aboveground
protection against decay for untreated wood, and for sections of
the wood that either did not take the preservative treatment
(heartwood) or for the interior of large cross-sections that were
exposed by cutting or drilling. The addition of the mildewcide
also prevents mold and mildew growth on wood treated with
waterborne preservatives, such as chromated copper arsenate
(CCA). Wood treated with CCA does not resist mildew fungi. The WR
and WRP finishes contain a water repellent, such as wax, and a
binder, but do not contain pigments. The binder, which amounts to
10 to 20% of the formulation, consists of drying oils (linseed or
tung oil) or a varnish binder. These oils or varnishes penetrate
the wood surface and cure to partially seal the wood surface.
They also help bind the multicide and make the wood surface water
repellent.
The WRPs that are formulated with nondrying oils that act as
solvents (such as paraffin oil) are also available. These oils
penetrate the wood, but do not dry. They protect the wood from
degradation and mildew attack, as do other types of WRPs. Because
the oils do not dry, the deck surface may remain oily until the
finish absorbs. This usually takes several days, but depends on
the application rate and porosity of the wood.
Penetrating finishes are extremely effective in stopping the
adsorption of liquid water, thereby decreasing dimensional
changes in the wood. Less dimensional change results in decreased
splitting, cracking, warping, and twisting, and less stress on
fasteners; therefore, less nail pull-out. These finishes enhance
the appearance and service life of both naturally decay-resistant
wood species and species pressure treated with wood
preservatives.
Several commercial wood treatments are using a WR treatment
combined with CCA treatment for 5/4 radial-edged decking. This
lumber is marketed under brand names such as Ultrawood, Wolman
Extra, MELCO, and Weathershield. With this dual treatment, wood
is more resistant to weathering. Although the WR should
thoroughly penetrate and saturate the wood, it is still advisable
to treat the ends that are cut during construction with a WRP. Of
the treated wood currently available, these treatments should
improve the wood characteristics and extend the product's service
life, particularly with sustained maintenance using a WRP.
Semi-Transparent Stains
When pigments are added to WRP solutions or to similar
transparent wood finishes, the mixture is classified as a
semi-transparent stain. Addition of pigment provides color and
greatly increases the durability of the finish compared with that
of the WRP. Semi-transparent stains permit much of the wood grain
to show through. The solvent-borne semi-transparent stains
penetrate into the wood without forming a continuous layer, and
consequently, will not blister or peel even if excessive moisture
enters the wood. The pigment protects the wood surface from
sunlight, thus increasing service life. The binder in the
solvent-borne oil-based, semi-transparent stain absorbs into the
wood surface, just as it does with the WRP, and there is no film
formation. The durability of a stain system is a function of its
pigment volume concentration, resin type and content,
preservative, water repellent, wood species and the surface
characteristics, and quality of material applied to the wood
surface.
If the decking material was given a factory-applied WR or if it
was recently finished with a WRP, a semi-transparent stain may
not absorb properly. In these situations, the wood should be
allowed to weather for two to three months before finishing. This
is the only situation in which it is beneficial to wait this long
before finishing the wood with a penetrating stain. Lumber should
not be left unfinished for six months to one year as indicated by
some product literature, or as recommended by some paint and
lumber suppliers. A short drying period may sometimes be
necessary.
Film-Forming Finishes
Film-forming finishes cover a wide range of finishes from
waterborne, latex-based, semi-transparent stains to paints, and
include both oil-based and latex solid-color stains (also called
opaque, full-bodied, or hiding stains). Almost all of these
products are unsuitable for use on wood decks. The only exception
is good quality paint, but even that can fail prematurely and
cause decay in the wood.
On structures that are fully exposed to the weather, such as
decks, paints tend to trap moisture and can actually increase the
decay hazard. The paint seal breaks at the joints between
different pieces of wood. These cracks permit water to enter the
wood and become trapped by the paint film. This leads to decay of
untreated wood, and the paint peels at these joints. Proper
pressure treatment with a preservative can eliminate the decay
risk. However, it is best to avoid the use of film-forming
finishes on lumber that is fully exposed to the weather, even if
the lumber has been pressure treated.
Conditions That Affect The
Finish..........(Return to Table of Contents)
Weathering Wood
Weathering of wood is a photochemical degradation of the surface
caused by the combined effects of water, the ultraviolet (UV)
radiation in sunlight, and abrasion by wind-blown sand.
Weathering should not to be confused with decay. Photochemical
degradation is manifested by an initial color change, followed by
the loosening of wood fibers and gradual erosion of the wood
surface. Rain washes the degraded wood materials from the
surface. Rain and/or changes in humidity cause dimensional
changes in the wood that accelerate this erosion process. In
softwoods, erosion is more rapid in the less dense earlywood
rather than in latewood, which leads to an uneven surface.
However, surface erosion proceeds slowly. The erosion rate for
solid softwoods, exposed horizontally in temperate zones, is on
the order of 5/16 to 5/8 in. (7 to 14 mm) per century, and
depends mainly on the intensity of UV radiation and the wood
species.
Weathering is a surface deterioration and, therefore, directly
affects finish performance. A film-forming finish applied to the
eroded and roughened surface of weathered wood may not adhere
properly; exposure of wood for only a few weeks is sufficient to
cause this loss of adhesion. Wood finishes also undergo
photochemical degradation, but the mode of degradation is
different for penetrating finishes (stains and water repellents)
than for film-forming finishes (paints).
The weathering of penetrating finishes, such as semi-transparent
stains and WRPs, is similar to unfinished wood. The surface of
the wood degrades, but at a slower rate than unfinished wood. If
the finish contains a pigment (semi-transparent stain), it
partially blocks the sunlight. As the wood surface and the finish
undergo simultaneous degradation, the pigment particles no longer
bond. As the pigment erodes from the surface, the degradation of
the wood increases. To avoid excessive wood degradation, timely
refinishing is essential. The surface should be refinished when
pigment loss is evident. The subsequent finish should absorb into
the wood. Finishing too early leads to inadequate absorption and
film formation. Finishing too late allows excessive wood
degradation. Pigment, film-forming finishes (e.g., paints) block
the damaging UV radiation and protect the wood surface. The
degradation of the film occurs on the surface of the film and
results in slow finish erosion. This is the most benign mode of
paint degradation because, as the primer coat begins to show,
another top-coat can be applied.
When painted wood of a deck is fully exposed to the weather,
another type of degradation occurs. This degradation mode
involves checking, cracking, and peeling, and can occur with
other film-forming finishes as well. When paint has peeled, the
surface of the wood is completely vulnerable to weathering. Paint
peeling is the most serious mode as paint degradation and is
likely to occur on painted decks.
Preservative Treatment
Finish characteristics of preservative-treated wood are primarily
dependent on wood species and grain orientation (flat or vertical
grain) of the lumber. Preservative-treated lumber is not graded
according to grain orientation. Therefore, lumber from a species
group like Southern Pine generally contain wide bands of
latewood, which can create finish problems. Stain penetration is
poor on these latewood bands, and early finish failure is more
likely to occur, particularly in structures that are fully
exposed to the weather.
Waterborne preservative treatments do not adversely affect the
finishing characteristics of the wood. In fact, CCA treatment
actually enhances the durability of semi-transparent stains and
similar finishes. CCA contains chromium oxides that bond to the
wood after treatment. These chromium oxides decrease
photodegradation of the wood surface, and can increase the
durability of semi-transparent stains two to three times (Cassens
and Feist, 1991). It should be noted that ammoniacal copper zinc
arsenate (ACZA) and copper oxide/quaternary ammonium compounds
(ACQ) do not contain chromium oxides; therefore, stain
performance on the type of treated lumber will be similar to that
on untreated wood given similar finish coverage and exposure
conditions.
Other preservative pressure treatments for wood, including
creosote and pentachlorophenol (penta) in light or heavy oils are
intended for use on retaining walls, railroad crossties, and
other heavy industrial applications. They should not be used on
wood decks. Penta in heavy oil and creosote will not hold paint,
so the lumber does not absorb semi-transparent stains well. Penta
in light oil or solvent can be finished after it weathers for a
year or two.
Moisture Content..........(Return to
Table of Contents)
The main consideration in finishing pressure-treated lumber is
the moisture content of the wood. In some cases, the lumber may
still be wet from the pressure treatment when it is delivered to
the job site, particularly during cool or wet weather. If the
wood is still wet, it must be allowed to dry before finishing so
that the surface moisture content is less than 20%. The rate of
drying depends on weather conditions; under warm, summer
conditions, about two to three weeks should be sufficient. For
the best finish performance, the wood should have a moisture
content that is typical of that which it will have during its
service life. This is about 12% for most areas of the United
States. (FPL, 1987)
Methods For Finishing New
Decks..........(Return to Table of Contents)
The first finish on wood, whether it is a deck or any structure,
is the most important. This finish should be applied as soon as
the wood surface is dry. If the first finish is not applied
properly, there is often little that can be done to correct
problems that develop later. The choice between a penetrating and
a film-forming finish should be given considerable study, because
it is necessary to continue with that type of finish in the
future.
If a penetrating finish is to be used, the next decision is
whether or not to use a semi-transparent stain or a WRP. The
service life of WRPs is about one year on the exposed surfaces;
however, WRP's are extremely easy to reapply to decks. They
absorb readily into the end grain of lumber and can stop the wood
from absorbing water into the end grain much longer than one
year. Because they are not pigmented, problems with uneven wear
and lap marks are eliminated. Lap marks occur during finishing
when the finish that is being applied to an area laps over an
adjacent area that has already been finished. This yields areas
that have two coats of finish, whereas the surrounding area has
only one coat.
To avoid lap marks in applying semi-transparent stains, brush the
stain on only two or three boards, being careful to keep the edge
of the stain along the edge of the board. Stain the entire length
of these boards. If a second coat is desired, it must be applied
while the first coat is still wet (within 30 to 45 min), or it
will not absorb. If the first coat has dried, it seals the
surface and the second coat cannot be absorbed. Brush the stain
onto the adjacent two or three boards, being careful to avoid
lapping stain on the boards previously stained.
If you are unsure whether to stain or to use a WRP, apply the WRP
on the deck first. It is possible to switch to a semi-transparent
stain when the deck needs to be refinished, Even if the deck has
been maintained with a WRP for many years, the semi-transparent
stain will perform satisfactorily, because it penetrates the wood
and will not fail by peeling, as can film-forming finishes.
The easiest finish to maintain on a deck is a WRP. The next best
is a semi-transparent, oil-based stain. Film-forming finishes,
such as paints, are prone to fail by peeling and are not
recommended. The WRP requires frequent application, but the ease
of refinishing compensates for the additional applications.
Conclusion/Summary..........(Return to
Table of Contents)
Long-term performance of horizontal wood structures exposed to
the environment and foot traffic is improved if maintained with a
finish after construction. The role of the finish is to protect
the surface from the natural weathering processes. Lumber should
be finished as soon as moisture content conditions permit;
additional weathering beyond necessary for drying will not
improve finish performance. The surface should be refinished when
some pigment loss is evident. Premature and delayed finishing can
lead to inadequate absorption or excessive wood degradation,
respectively. Ease of finishing should be considered in selection
process. Film-forming finishes, such as paints are generally not
recommended for horizontal exposures because water can be trapped
and actually improve the conditions for decay fungi.
References..........(Return to
Table of Contents)
Cassens, D.L., and W.C. Feist. 1991. "Exterior Wood in the
South." General technical report, FPL-GTR-69. USDA Forest
service, Forest Products Laboratory, Madison, WI, 56p. Forest
Products Laboratory (FPL). 1987. Wood handbook: Wood as an
engineering material. Agricultural Handbook no. 72. USDA,
Washington, DC. 466p.
Williams, R.S., Supervisory Research Chemist, and Feist , W.C.,
Research Chemist, USDA Forest Service, Forest Products
Laboratory, Madison, WI. The Forest Products Laboratory is
maintained in cooperation with the University of Wisconsin. This
article is written and prepared by U.S. Government employees on
official time, and is, therefore, in the public domain and not
subject to copyright.
Webpage creation by Dan Mitchell, working in
the Historic Home Works Assistantship Program. Dan is a
preservation enthusiast on Cobbosseeconte Lake in Maine.
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|
John Leeke's Historic HomeWorks™ |
|
207 773-2306 26 Higgins St. Portland, ME 04103 |
|
| [Home][Library][Restoration Reports][Seminars][Forum][Internships][Office][Workshop][Front Porch][Search] info.© 1994-2009 JohnC.Leeke |