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(Return to the List of Articles)
"The Changing Nature
Of Wood Products: What Does It Mean For Coatings And Finish
Performance?"
Joint Coatings/Forest Products Committee
Chairman: Charles Jourdain, California Redwood Association,
Novato, CA (1-415-382-0662) Jack Dwyer, VT Dept. of Forests and
Parks, Waterbury, VT Keith Kersell, The Pacific Lumber Company,
Scotia, CA Douglas Mall, Tru-Test Manufacturing Co., Cary, IL Ken
McClelland, Western Red Cedar Lumber Association, Vancouver, BC
Robert Springate, Tru-Test Manufacturing Co., Chicago, IL Sam
Williams, USDA Forest Products Laboratory, Madison, WI
Introduction..........(Return to Table of Contents)
Most current recommendations concerning the application and
performance of coatings and finishes on exterior solid wood
substrates are based upon knowledge gained decades ago. Chapter
16 of the Wood Handbook: Wood as an Engineering Material (1),
discusses the properties of wood and the interaction of these
properties with wood finish systems. A major part of this
discussion is the result of research conducted by Browne (2) on
wood properties and paint durability during the decades of the
50's and 60's. Since that time, dramatic changes have occurred to
both the country's timber base (3) and to exterior wood finish
formulations. The shift from exterior wood products originating
in the old growth forests of the Pacific Northwest (British
Columbia, Washington, Oregon, and Northern California) to
products originating in the vigorous second and third growth
forests of the northeastern, mid-western, southern United States
and eastern Canada, as well as the Pacific Northwest, has
accelerated in the past decade. The species mix in almost all
areas of the United States is vastly different today compared
with 50 years ago. Environmentally driven restrictions on certain
chemicals and volatile organic compounds (VOCs) have resulted in
a reformulation of nearly all paints, stains and natural wood
finishes during this same time frame. The objective of this paper
is to examine how the changing timber base has affected the
characteristics of the products derived from that base and how
these characteristics affect the performance of the new
generation of wood finishes.
Old growth timber is generally defined as being 200 or more years
of age. Lumber products derived from such a resource are
characterized by high percentages of clear (free from knots and
other defects) grades, with very fine, uniform rings of annual
growth. Another natural consequence of the manufacture of lumber
from the larger logs typical of old growth is the yield of
relatively high percentages of heartwood grades and lumber with
vertical grain (i.e., quartersawn). Today's commercial forests
are typically harvested at an age that may range from 20-30 years
for plantation-grown southern pine to 60-70 years for west coast
species such as redwood and Douglas-fir. A result of harvesting
timber from such forests is a difference in the characteristics
exhibited by the wood. Smaller diameter, more rapidly grown trees
yield higher percentages of flat-grained lumber containing knots
and other natural characteristics including sapwood. Restrictions
on harvesting in west coast forests has given more interest to
other regions of North America. Species harvested from these
other areas may not be as well suited for exterior applications
as redwood and western red cedar.
Wood Properties..........(Return to Table of Contents)
Wood properties affect the efficiency and methods of application
and service life of finishes (finishability). These properties
include natural properties such as density, growth rate, knots,
extractives content, and juvenile wood and manufacturing
characteristics such as grain orientation, surface texture, and
moisture content. These properties were discussed previously.
(4,5). The link between the properties of wood and its
finishability developed over 75 years by numerous researchers
from many laboratories remain valid today. For example, for a
specific species and grain orientation, the effect of density on
the weathering rate of wood remains the same whether the wood is
obtained from an old growth tree or a fast-grown plantation tree.
The weathering rate/density relationship is the same, but the
wood obtained from the fast-grown tree tends to be less dense. It
is the changes in the tree size, growth rate, and species mix of
the trees being harvested, changes in manufacturing practices,
and a greater reliance on wood composites that has led to vastly
different substrates, not any inherent differences in the
properties of a given tree species. These recent changes must be
superimposed on the properties described previously. (4,5)
Exterior Wood Products..........(Return to Table of Contents)
Architectural Products and Species
The vast majority of wood used in residential and nonresidential
construction is used for the internal structural framework and,
therefore, does not need a protective coating and/or finish
system. Wood and wood products are also used to protect the
structure and are exposed to the weather. Exposed wood may
include architectural applications such as siding, facia, trim,
doors and windows, and landscaping elements such as decks,
trellises, gazebos and fences. These exterior wood products are
usually protected by various finish systems. These systems will
enhance the long term performance and appearance of wood exposed
to the weather.
Considering only solid sawn wood products as opposed to composite
products such as plywood, hardboard oriented strandboard (OSB),
etc., a wide variety of species may be used in exterior exposure
as architectural or landscaping elements. These elements are
typically constructed from softwood (coniferous) species such as
pine, fir, cedar or redwood. Premium wood siding, trim or facia
is usually redwood or cedar. These species have heartwood that
possess natural resistance to wood-destroying organisms such as
fungi and termites, as well as outstanding inherent dimensional
stability. Redwood, cypress, and the various species of cedar
have traditionally been rated as those species with the best
characteristics for painting and weathering. (1) Fir, pine, and
to a lesser extent, hemlock and spruce, may also be used for
exterior architectural and landscape purposes. Pressure
preservative treatment with waterborne arsenicals such as
chromated copper arsenate (CCA) is commonly done with these
species to provide decay and termite resistance. These species
all rank below vertical-grained redwood and cedar in their
ability to hold paints. (1)
Imported Wood Species
The decade of the 1990's has been a time of unprecedented change
in the wood resource within North America. Several factors have
come into play to drive these changes. In addition to changes in
the size and species mix of the log harvests, there is an
increase in log imports into the USA. Radiata pine from Chile,
Australia and New Zealand and a variety of species from the
former USSR (larch, pine, and spruce) are being imported.
Although radiata pine has been used for many years in other
countries, and its properties are well known in those countries,
users within the USA are not familiar with this wood. Radiata
pine grows extremely fast often giving one to three growth rings
per inch. The logs from countries of the former USSR will likely
be small diameter and slow grown given the latitude and climate
conditions for those forests.
Increased imports of tropical hardwoods should be anticipated.
The density, machinability, weathering properties, paintability,
and extractives bleed of these species, have not been fully
researched. Many of these species are high in extractives and
oils and it is not known whether paint formulations available in
the USA and Canada will block them. Paint companies generally do
not test formulations on tropical wood species.
Another change in the lumber resource is the small but growing
amount of recycled wood being used. This wood may be available
from a wide variety of sources, have many different species, and
be used in a variety of ways. In some cases, large timbers are
being used as is or they are being sawn into lumber. Some of this
wood is being use in composite materials.
Silvicultural Practices To Enhance Wood Quality
Silviculture is the practice of controlling the establishment and
development of stands of trees to achieve the objectives of
management. Most commercial timber production is primarily
concerned with maximizing the volume of wood fiber produced per
acre per year with secondary consideration being the quality of
the fiber. There are ways, however, to improve quality while at
the same time not sacrificing the primary objective. Intensive
management practices such as pruning can result in the formation
of more clear lumber. Deferred rotations allow the best quality
trees to add greater volumes of high value clear vertical-grained
lumber. Growing trees to larger sizes and older ages, however,
involves significant financial risk that some landowners might
not be willing to take.
In addition to the physical manipulation of trees and stands, the
genetic selection of "super trees" can also improve
lumber quality. Trees can be selected for certain genetic
characteristics such as density, juvenile wood content, heartwood
content, and branch angle. While much research has been conducted
to improve stands through selection and the development of
hybrids, the availability of genetic engineering techniques has
made available additional techniques for designing "super
trees". There is much more work to be done and significant
potential gains in lumber quality from genetic engineering may
take decades to achieve. Despite this, it is apparent that
silvicultural practices and genetic selection have had a
significant impact on sawn lumber grade yields from managed
forests, particularly in the increased amounts of fiber per year
per acre.
The primary result of these practices has been faster growth
rates that give lumber with wide bands of earlywood and latewood.
These wide bands become more problematic when the lumber is
flat-grained.
Lumber Grades and Quality
Lumber is generally graded according to structural grade rules or
appearance grade rules according to well defined characteristics
by various rule-writing agencies. Wood characteristics include
both natural characteristics such as the size and location of
knots and manufacturing characteristics such as chipped grain,
raised grain and skips in surfacing. All permissible
characteristics of a grade, such as knots, are never present in
maximum size or number in any one piece. Likewise, a particular
parcel of lumber will not consist of only pieces containing
characteristics of the maximum number or size permitted in the
grade.
Recently, much controversy has centered over the belief that the
quality of lumber grades is slipping. Several factors have
contributed to this widely held belief. Harvesting smaller
diameter young growth trees gives a greater proportion of common
or knotty type grades in the lumber. A much smaller proportion of
the clear higher grades are recovered from today's resource. The
result is a significant shift in the recovery ratio of clear
grades to common grades. Twenty-five years ago, clear grades may
have represented up to forty percent of the lumber produced.
Today, only ten to twenty percent of all lumber production gives
clear grades. In the past, when an order of No. 2 and Better was
delivered, a healthy percentage of the boards would be
"Better" because the time it took to sort through and
separate them from the others was worth less than the price
differential. That is no longer the case. (6)
The allowable characteristics in a particular grade have not
changed dramatically over the last twenty-five years. An analysis
of the grade descriptions in the various species grade rule books
from twenty years ago reveals descriptions very similar to those
published today. What will actually be found in today's lumber
products are a higher percentage of pieces that possess more
numerous maximum allowable characteristics for the particular
grade in question than in the past.
Two changes have occurred in appearance grades of lumber, which
often include siding and trim boards. These are changes in
dimension and a greater reliance on finger-jointed lumber.
Product dimension. To increase yield, some manufactures have
decreased the thickness of siding and other products. The cross
sectional profile of a product (the aspect ratio of thickness
versus width) can significantly affect product performance.
Thicker narrower products will perform better from a dimensional
stability standpoint than thinner wider patterns or products.
There is often a choice of product dimension for a particular use
and the specifier (architect, owner, contractor, etc) must select
a particular product. As an example, 3/4x6-inch bevel siding
would be expected to be a more stable product on a building's
exterior than 1/2x8-inch bevel siding. The consideration of
product dimension is even more important with today's lumber
products which are predominantly flat-grained and therefore
inherently somewhat less dimensionally stable than
vertical-grained products.
Finger-jointing and edge-gluing. Finger-jointing and edge-gluing
have been utilized for decades to increase recovery of clear,
knot free lumber (Figure 1). The amount of glued product using
these manufacturing techniques has increased dramatically over
the past decade as the need for clear lumber increased and the
amount of clear sawn lumber has decreased because of the harvest
of smaller diameter trees. The advantage of finger-jointing and
edge-gluing is that longer and wider knot free boards can be
produced from a young growth resource that otherwise would
produce a large proportion of lower quality material. Glued
products also significantly increase the dimensional performance
of a board by decreasing the woods' natural tendencies to shrink
and warp. Finger-jointed and edge-glued boards stay straighter
and perform better than solid sawn lumber as a result of their
improved dimensional stability. These advantages are more obvious
in glued lumber made from vertical-grained stock where the
differences in grain from piece to piece are less noticeable.
Vertical-grained finger-jointed lumber provides a more uniform
substrate for finishing, eliminating the wide variation in grain
associated with flat-grained lumber and avoiding the poorer
quality wood fiber found near knot locations.
While flat-grained finger-jointed lumber also eliminates problems
at knot locations, it will also tend to have more grain raising.
The short stubs used to manufacture flat-grained finger-jointed
lumber usually exhibit a wide range of texture, grain angle, ring
count, and other physical characteristics. As a result, lumber
made from flat-grained finger-joint stubs tends to be non-uniform
in appearance and performance. Sometimes, little care is used to
match grain patterns, growth rate, or heartwood/sapwood during
manufacture. Each piece may then contain both vertical-grained
and flat-grained sections. The characteristics of this lumber are
controlled by the lowest quality component, i.e. the flat-grained
portions. The service life of the finish is determined by these
flat-grained sections. It is not uncommon for spotty failure to
occur only on the flat-grained portions, often within a year or
two, while the vertical-grained portions last for years. For this
reason, saw-textured surfaces or rough-sanded surfaces are highly
recommended for flat-grained glued products to help decrease the
effects of the wood variables on the appearance and performance
of the finished product.
Flat-Grained Siding and Lumber- Affect on Finishing
It is well known that the paint holding characteristics of
latewood are not as good as those of earlywood. In slow-grown
vertical-grained lumber, particularly with species having narrow
latewood bands (such as redwood and cedar), these latewood bands
are easily bridged by the coating and there is little stress
induced in the coating at the latewood/earlywood boundary. The
amount of latewood in contact with the coating system is small
and there is little raised grain or other dimensional changes at
the latewood/earlywood interface. In wood species having thick
latewood bands (such as southern pine), the bands of flat-grained
lumber, can be quite wide. In some cases, more than an inch
(25mm) and the difference in density between the earlywood and
latewood at the interface can lead to severe grain raising and
stress on a coatings system. The changes in a surface are shown
in the two book-matched cross-section of a board following
planing before and after wetting with water. (Figure 2) Note that
the latewood bands were crushed into the less dense earlywood
bands during planing and then rebounded when wet to give the
raised grain. This type of raised grain is usually more severe on
the pith side and it can easily crack a coating system,
particularly an alkyd paint system. (Figure 3, 4)
The effect of surface quality, and texture play an important role
in the performance of a finish. Saw-textured (also referred to as
"resawn"), sanded, and smooth-planed surfaces all
influence finishes differently. Generally speaking, saw-textured
surfaces accept more finish than smooth-planed surfaces and thus
the finish lasts longer. These roughened surfaces provide more
tooth for finish adhesion and actually have much more surface
area. These types of surfaces also dramatically decrease grain
raising problems associated with flat grain lumber by decreasing
grain compression and other damage sometimes caused by planing
processes. As the timber resource changes from an old growth to a
young growth supply, there is less vertical-grained wood
available. For this reason, saw-textured surfaces are being used
more frequently in an effort to decrease grain raising, and
improve finish performance on flat-grained wood products.
Saw-textured surfaces greatly decrease problems with raised
grain. For flat-grained lumber or siding, the single best
procedure for finishing is to begin by using the saw-textured
side. Other suggestions are included in the section on painting.
A special problem exists with lap or bevel siding. This siding is
manufactured by ripping a piece of dimension lumber on a diagonal
to form two pieces of siding. The side that is ripped is
saw-textured. If the original board is vertical-grained, both
pieces of bevel siding will be vertical-grained and there is no
difference with regard to bark/pith side. If, however, the
original board is flat-grained, the two pieces of bevel siding
have opposite bark/pith orientations. One piece will have bark
side grain on the smooth side and pith side grain on the resawn
side, whereas the other will have pith side grain on the smooth
side and bark side grain on the resawn side. It is not possible
to use only the bark side regardless of whether the smooth or
saw-textured side is used. Fewer problems will occur if the
saw-textured surface is used.
Flat-grained lumber often has knots. There is no easy solution
for avoiding cracking of the finish and bleed-through of
extractives and/or resins. The amount of extractives/resins
available from the end-grain and the diversity of these chemicals
make blocking them extremely difficult. The only way to seal the
knots is by using shellac or a comparable commercial product.
These products tend to be alcohol based and are therefore water
sensitive. They should only be used over the knots, not on the
whole board. Their use adds another step to the finishing
process. It might be better to recognize that the knots are a
characteristic of the wood and one that can add beauty. Instead
of working against nature and trying to cover them, selection of
a finish system that works with the knots might be an easier
alternative. White paint and other light-colored film-forming
finishes are extremely difficult to use on knotty wood. If the
knots are large, they will eventually crack. These cracks will
cause any film-forming finish to crack; alkyds are more
susceptible to cracking than are latexes. Suggestions for
finishing knotty wood are contained in the section on painting.
Wood/Wood Composites
Wood based composites are being used to a greater extent than in
past years. Traditional wood based composites will continue to be
used for many structural and decorative components. These include
plywood, hardboard, oriented strand board (OSB) and various types
of particleboard. Many will include veneers and/or paper-overlaid
products for the surface. Panel products will continue to be used
because of their competitive pricing and ease of installation. A
number of siding materials using wood furnish of flakes, strands,
particles, and fibers are being produced with a paper-overlaid
surface. These products have an excellent surface for painting,
however edge sealing will continue to be a challenge for these
materials. These products are usually produced in large sheets,
which may then be cut to give siding of various styles. These
products are often edge sealed during manufacture, but field
application of a paint to ends cut during construction is
necessary to give protection against water. Species mix will
change with increasing dependence on temperate hardwoods such as
aspen, red maple and yellow poplar , tropical hardwoods, and
radiata pine. Research on wood composites and adhesives for wood
composites is continuing and, almost every year, new products are
being introduced into the marketplace. The burden for assuring
their compatibility with finishes often falls on the coatings
industry.
Wood composite products often lack the robustness that is
characteristic of solid wood products. Whereas solid wood
performs amazingly well in spite of abuse during shipping,
storage, and handling at the construction site, wood based
composite products may fail prematurely if not handled properly.
When these products are used in accordance with manufacturer's
instructions, they often perform quite well for decades. For
example, plywood siding, particularly siding having a
saw-textured surface, lasts for decades when protected by a
finish system. Plywood can give excellent performance even
without a paper overlay, however almost all other wood/wood or
fiber-based composites perform best if they are manufactured with
a paper overlay.
Wood/Plastic Composites
Wood/plastic composites such as decking, pallets, furniture,
automotive parts, etc. have become available during the last five
years. For example, one wood/plastic decking board is a
wood-fiber/polyproplylene extruded 2x6. Although this material
has made some market gains since its inception, it lacks the
thermal, flame resistance, and mechanical properties necessary to
replace dimension lumber. These materials initially used wood
fiber as an inexpensive source of filler, however this is
changing. As chemicals are developed for modifying the
fiber/plastic interface, these materials will be engineered for
greater strength, stiffness, and toughness. Efforts are
continuing to improve products such as these, and it is
anticipated that similar products will be introduced to be used
as siding materials, sheathing, decorative moldings, etc. in the
near future. Some of these materials are already available for
interior use.
These wood/plastic composites will also be made using substitutes
for the wood fiber. Many other fiber sources are being
investigated for use in these materials, such as jute, flax, and
hemp. Considering the vast array of fiber types and polymers,
their properties, and the different manufacturing methods, the
range of products available seem almost infinite.
Paper-Overlaid Products
Paper-overlaid products are usually manufactured by bonding a
resin-impregnated kraft paper to the surface of the composite
during manufacture. The paper eliminates surface defects and
makes the overlay surface less moisture sensitive. The edges must
still be protected and this is the weak point for all of these
products.
Fiberboard
The surface of fiberboard, whether overlaid with paper or not,
has a homogeneous surface and accepts paint rather well. As with
other composite siding materials, the edges are moisture
sensitive, and must be protected with a finish.
Painting Guidlines..........(Return to Table of Contents)
The painting guidelines in this section are neither
recommendations, nor are they intended to be the only method that
would give good protection to the wood. Almost every situation is
different. The design of the building, construction practices,
customer/owner preferences, cost, and weather conditions make it
impossible to make finishing recommendations that can apply for
all situations. Using knowledge of the properties of wood, the
use conditions, customer/owner preferences, and costs, the
painter must make the final decision. This decision often
involves compromises. This section includes mainly a discussion
of challenging areas; it is not a comprehensive guide to wood
finishing. Since much of the changing resource relates to
flat-grained lumber, finger jointed lumber, and composites, these
areas are stressed.
Flat-Grained Solid Lumber and Siding
The single most important method for improving finish performance
on flat-grained lumber and siding is to finish a saw-textured
surface. If the lumber does not have a saw-textured surface,
scuff sand it with 50-grit or rougher sandpaper. If a
film-forming finish is to be used, an all-latex system is best.
However if highly colored wood, such as redwood or cedar are
used, the primer must be a stain blocking primer.
Siding having knots can be divided into two groups, those having
small inter-grown knots, and those having large knots. The
division between these two groups is not clear in terms of size,
but in the property of the knot. If the knot can split, then it
is large; if it can not split, then it is small. The guidelines
for finishing depend on this distinction. Siding having small
inter-grown knots can be finished with either a film-forming
finish or a penetrating finish. Since it is almost impossible to
eliminate discoloration, avoid the use of light-colored finishes.
If the knots are large, finish the wood with a penetrating
finish, preferably one that is pigmented to enhance the knots.
Finger-Jointed Lumber and Siding
The only difference between the flat-grained lumber described
above and finger jointed lumber is the absence of knots. Although
some manufacturers sort by grain angle to produce
vertical-grained and flat-grained products, the vast majority of
manufacturers do not sort by grain angle during the manufacture
of finger-jointed products. Therefore, unless the finger-jointed
product is specifically marked as vertical-grained, all boards
must be treated as being flat-grained. As with other flat-grained
products, the resawn side will perform best. Semi-transparent
stains are suitable for this substrate and all latex paint
systems perform better than oil/latex or oil/oil paint systems.
Boards will have a mix of pieces with various extractives content
and resins. It is imperative that a stain blocking primer be
used. Even with slightly colored species, such as pine, the
resins can cause discoloration with light-colored finishes. It is
best to avoid the use of light-colored finishes.
Plywood
Plywood is a flat-grained surface, therefore finishing guidelines
are similar to those described above for flat-grained lumber.
Plywood panel products intended for exterior use are often given
a saw-textured surface. These products can either be stained or
painted and give far superior performance to smooth-planed
plywood. Best paint performance is obtained from saw-textured
surfaces using all latex paint systems; penetrating
semi-transparent stains also give good performance on these
saw-textured surfaces. Plywood having a paper overlay, cannot be
finished with a penetrating finish such as a semi-transparent
stain. Film-forming finishes will perform better on these
paper-overlaid products than on the non-overlaid plywood,
particularly if the surface is not saw-textured.
Wood/Wood Composites
Other than plywood and fiberboard, wood/wood composites should
have a paper overlay if they are used outdoors. The edges must be
protected from moisture with a coating. The paper-overlaid
surface cannot be stained, but it can be finished with any type
of film-forming finish.
Wood/Plastic Composites
Wood/plastic composites are generally not designed to be painted
or stained. The paints are likely to debond and stains do not
absorb properly. Those products used for decks tend to weather,
become porous, and develop mildew. Cleaning and application of
WRP, after they become weathered, improves their performance.
Paper-Overlaid Products
Paper overlay of wood composites (including plywood) provide an
excellent surface for painting and the overlay eliminates grain
raising, extractives bleed and problems with knots. The
paper-overlaid surface of these materials cannot be stained with
solvent-borne semi-transparent stains or treated with
water-repellent preservatives. However, improved performance is
obtained if the edges are treated with a WRP prior to being
painted.
Fiberboard
Fiberboard, whether overlaid with paper or without a paper
overlay, can be finished with any type of film-forming finish. If
not overlaid with paper, take care to use a primer that will
block extractives. These substrates cannot be finished with
semi-transparent stains.
Factory Priming and Finishing
Finishes adhere to the wood substrate and perform better when
they are applied under ideal conditions. Generally speaking,
finishes should be applied to dry wood, at a moisture content of
10-15% and at temperatures above 50 degrees Fahrenheit. These
conditions are not always present during field application. To
help avoid poor finishing situations many lumber manufacturers
are offering factory applied primers and finishes. Factory
applied primers and finishes are usually more uniform because of
better quality control during application. Substrate and drying
temperatures, wet and dry mil thickness, finish viscosity, and
line speeds are regulated with a much higher degree of accuracy
than can be accomplished in field applications. The result is a
better performing, more economical coating than one manually
applied at the job site sometimes under adverse weather
conditions.
Summary and Conclusions..........(Return to Table of Contents)
As the fundamental nature of North American forests have changed
over the past 200 years, so has the properties of the building
products derived from those forests. Smaller diameter second and
third growth trees yield lumber with higher percentages of knots
and sapwood and less of the clear fine grain lumber traditionally
used for architectural applications. Many of the current
recommendations for finishing exterior wood products are based
upon dated information. The more knotty solid wood products used
today may have coarser grain and be predominantly flat-grained.
Flat-grained lumber is more prone to grain raising and
dimensional change. Saw-textured surfaces decrease the potential
for grain raising and they enhance finish performance.
Saw-textured knotty flat-grained wood products may perform better
if they are finished with penetrating finishes (semi-transparent
stain) rather than with film-forming finishes (paints).
Semi-transparent stains and other types of penetrating finishes
do not peel and tend to be less likely to show knot or
extractives bleed. Film-forming finishes may also be used on
knotty saw-textured flat-grained surfaces, however discoloration
because of bleed-through at the knots continues to be a
challenge, particularly with light-colored finishes. Use of
different wood species, flat-grained knotty wood, and composites
may require some changes in finishing practice and more careful
selection of products for specific uses. For example, if a white
painted look is desired, it may be better to use a paper-overlaid
product to eliminate problems with knot bleed than to try to seal
the knots using shellac. If finger jointed lumber is used, extra
care may be required to assure that the finish can adequately
block the extractives. Sealing and painting the edges of
composites is necessary to assure good performance.
Paper-overlaid products give a good surface for painting.
Advances in finger jointing and factory prefinishing will add to
the performance of modern wood products. In the coming decade,
the transition to lumber from rapidly grown intensively managed
forests will continue. New species and composites will become
more available in the marketplace. Much of the burden for
assuring the compatibility of finishes with new wood products
will probably rest with the coatings manufacturers and their
suppliers. The technology of paints and other finishes and the
protocols for testing finishes must advance to keep pace with the
changes in the substrates.
References..........(Return to Table of Contents)
1. Forest Products Laboratory. 1987. Wood Handbook: Wood as an
Engineering Material. Agriculture Handbook 72. USDA, Washington,
D.C.
2. Browne, F.L. 1951. "Wood Properties That Affect Paint
Performance." USDA Forest Service. FPL Report No. R1053.
3. MacCleery, D.W. 1992. American Forests: A History of
Resiliency and Recovery. USDA Forest Service. FS-540.
4. Williams, R.S., M.T. Knaebe, and W.C. Feist. 1996. Finishes
for Exterior Wood. Forest Products Society, Madison, WI. 125 pp
5. Jourdain, C., D. Dwyer, K. Kersell, D. Mall, K. McClelland, R.
Springate, and S. Williams. 1999. "Wood Properties Affecting
Finish Service Life." Journal of Coatings Technology.
6. Hunt, R.H. 1994. "Lumber Quality Not Declining."
Building Products Digest. Volume 13, No. 7.
Figure captions
1. Finger-jointed lumber
2. Raised grain on book-matched specimens of planed lumber. The
cross-sections of a planed board illustrate grain-raise caused by
water. Surface immediately following planing but not wetted
(top); the same planed surface but wetted (bottom).
3. Raised grain on the pith side of a board (left), and the bark
side of the same board (right) without raised grain.
4. Paint failure caused by raised grain.
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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