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Wood Properties Affecting Finish Service Life
Educational Feature
Wood is a biological mate-
rial that has widely different
properties depending on spe-
cies, geographic area where
R. Sam Williams—USDA Forest Service*
the tree grew, the growth
Charles Jourdain—California Redwood Association**
George I. Daisey—Rohm and Haas Co.† conditions, size of the tree at
Robert W. Springate—Tru-Test Manufacturing Co.†† harvest, sawing, and other
manufacturing processes.
C
urrent knowledge on wood prop- though wood properties themselves
erties has accumulated through have not changed, finishing practices Some of the more important
decades of research in laborato- need to accommodate changes in spe- wood properties as they re-
ries located in many parts of the world cies mix, the dependence on small, fast-
and is contained in a number of publica- grown second and third growth timber, late to wood finishing are
tions.1–7 Much of this knowledge, par- and more widespread use of wood com-
ticularly information on North Ameri- posites to meet lumber demands. Flat- discussed, e.g., growth rate,
can wood species, is summarized in the grained lumber produced from small- density, knots, extractives,
Wood Handbook: Wood as an Engineering diameter, rapidly grown trees contains
Material.8 Detailed information on wood a higher percentage of knots, wider juvenile wood, grain orien-
properties that relate to wood finishing growth rings, and possibly juvenile tation, and weathering char-
is given in Chapter 15 of that publica- wood. The ways that these properties
tion, in Finishes for Exterior Wood,9 and in affect finishing practices are discussed acteristics.
a monograph by Feist.10 The benchmark elsewhere.13 This paper is an overview
research conducted by Browne11 during of wood properties. Wood that has been
the 1950s and 1960s is still valid and chemically treated (by preservatives, fire
stands out as the authoritative work retardants, or bulking agents) and wood properties of these wood-based materi-
documenting the effect of wood proper- composites (plywood, flakeboard, and als will be the subject of an additional
ties on finish performance, particularly fiberboard) do not usually have the same publication.
oil-based film-forming finishes. Detailed properties as the original wood. The
information on wood properties is con-
tained in these publications. This paper
covers wood properties that have the Natural Characteristics
greatest effect on finish performance and
particularly the properties of lumber Growth Rate
available from today’s forests.
In temperate climates, tree species add
Although dramatic changes have oc- one growth increment, or ring, to their
curred to both the country’s timber base12 diameter each year (Figure 1). This ring
and to exterior wood finish formulations shows two distinct periods of growth
since the 1950s and 1960s, the relation- and therefore two bands, called early-
ships between finish performance and wood (springwood) and latewood (sum-
wood properties have not changed. Al- merwood). Latewood is denser, harder,
smoother, and darker than earlywood,
*Forest Products Laboratory, One Gifford Pinchot Dr.,
and its cells have thicker walls and
Madison, WI 53705-2398. smaller cavities. The proportion of ear-
**405 Enfrente Dr., Ste. 200, Novato, CA 94949. lywood to latewood, the density differ-
†
727 Norristown Rd., Spring House, PA 19477-0904.
††
823 W. Blackhawk, Chicago, IL 60622.
ences, and the transition from the less
dense earlywood to the more dense late-
The use of trade or firm names in this publication is for
reader information and does not imply endorsement by the U.S. wood during the growing season can
Department of Agriculture of any product or service. vary, depending on species and grow-
The Forest Products Laboratory is maintained in coopera-
tion with the University of Wisconsin. This article was written
Figure 1—Cross section of log ing conditions. Many species have a
and prepared by U.S. Government employees on official time, showing heartwood, sapwood, rather gradual change in density during
and it is therefore in the public domain and not subject to earlywood, latewood, and pith. the growing season. Some species have
copyright.
Vol. 72, No. 902, March 2000 35R.S. Williams et al.
added texture to the wood and some- a tree (Figure 4). The influence of knots
times make it more difficult to finish on the finishing properties of lumber
these species. depends on their size, location, shape,
and soundness. When sawing lumber
from a log, a nearly round shaped knot
Density results when a branch is sawn through
The density of wood is one of the at right angles to its length as in flat-
most important factors that affect finish- grained lumber. Vertical-grained lum-
ing characteristics. Density varies tre- ber may have spike knots, which result
mendously from species to species (Table from sawing a branch radially (Figure 5).
1), and this is important because high Oval knots will result from sawing a
Figure 2—Paint failure over wide density woods shrink and swell more branch diagonally as in lumber that is
latewood bands on plywood. than do low density woods. This dimen- intermediate between flat grain and ver-
sional change occurs as wood, particu- tical grain. Knots are further classified
larly in exterior applications, gains or as intergrown or encased. Intergrown or
rather narrow latewood bands; others loses moisture with changes in the rela- tight knots result from the growth of the
have wide latewood bands. For some tive humidity and from periodic wet- stemwood around a living branch (Fig-
species, the density differences are not ting caused by rain and dew. Excessive ure 4). After a branch dies, additional
very large; others have large differences dimensional change in wood stresses growth on the stem encloses the dead
in density between the latewood and film-forming finishes and may cause limb. This results in an encased knot
earlywood. However, for all temperate cracking or flaking, or both. where the fibers of the stem are not con-
species, the change in density from the tinuous with the fibers of the branch.
The amount of warping and check- Because shrinkage is greater across the
end of one growing season to the begin- ing that occurs as wood changes dimen-
ning of the next is always abrupt. If the knot than in the surrounding wood, en-
sion and during the natural weathering cased knots may loosen and fall out caus-
difference is large, the stresses imposed process is also directly related to wood
on a coating system can be quite large ing a knothole. Encased knots often have
density. Cupping is probably the most bark surrounding the knot, which may
when dimensional changes occur at this common form of warp for siding. Cup-
transition zone. This abrupt transition cause additional discoloration problems
ping is the distortion of a board that with light-colored finishes.14
in wood density also causes differences causes a deviation from flatness across
in properties for the pith side and bark the width of the piece. Wide boards cup Knots are usually considered defects
side of flat-grained lumber. more than narrow boards, and flat- in wood for a number of reasons. The
The dimensional changes at the late- grained boards cup more than vertical- wood of the knot itself is different in
wood–earlywood interface can cause grained boards. Boards may also bow, density (usually higher), and its grain
cracks in film-forming finishes at this crook, or twist from one end to the other. orientation is more or less perpendicu-
zone. Paint failure on latewood often Warping is generally caused by uneven lar to the surrounding wood. Grain ori-
begins with these cracks. If the bands of shrinking or swelling within the board. entation around the knot is often dis-
latewood are narrow enough, as in slow Furthermore, checks or small cracks torted, which makes it difficult to achieve
growth trees, the stresses are decreased along the grain may develop from stress an acceptable machined finish on the
and there is less tendency for paint to that developed during the initial drying surface of the wood. For structural lum-
crack or peel than on the wide latewood (shrinking) or from stresses caused by ber, the size and location of knots are
bands. Wide latewood bands are nor- the alternate shrinking and swelling dur- two of the factors that determine grade
mally absent from edge-grained cedar ing service. High density (heavy) woods and thus the design values of the lum-
and redwood, improving the paintability such as southern pine, Douglas fir, and ber. Knots also affect the appearance
of these species. It is well established oak tend to warp and check more than grades of lumber, which include siding
that wide latewood bands on softwoods do the low density (light) woods such as and trim. Although knots are usually
give a surface that is difficult to finish considered defects, a number of species
redwood and western redcedar (Table 1).
(Figure 2). They are prominent in south- and grades that exhibit knots with some
ern yellow pine and Douglas-fir, two of degree of regularity such as knotty pine,
the most common species used for gen- Knots select tight knot (STK) cedar, and rustic
eral construction and for the production grades of redwood have been success-
A knot is the portion of a branch that fully marketed to feature their knots.
of plywood. Coarse grain (i.e., wide an-
has become incorporated in the stem of
nual growth rings of alternating early-
wood and latewood) is a natural result
of the rapid growth rate of young-growth
forests under the optimal growing con-
ditions of intensive silviculture. On the
other hand, growth rate does not seem
to significantly affect the ability of hard-
woods to retain a film-forming finish.
They are generally more difficult to fin-
ish than softwoods (Table 1).
Paint performance on hardwoods,
such as oak, is more a function of den-
sity and the size and location of vessels
rather than distinctions between early- Figure 3—Wood anatomy affects the durability of paint. Left, a nonporous wood
wood and latewood (Figure 3). These (softwood or conifer such as southern pine); center, a ring-porous wood (white oak)
vessels are common to hardwoods only showing the large open vessels characteristic of this species; and right, a diffuse-
and are sometimes much larger than the porous wood (yellow-poplar).
surrounding wood cells. They give
36 Journal of Coatings TechnologyWood Properties Affecting Finish Service Life
Table 1—Characteristics of Selected Solid Woods for Painting and Finishing
Paint-Holding Resistance to Conspicuousness Degree of
Specific Characteristic Cupping of Checking Figure on
Gravity (I Best, V; (1, Most; (1, Least; Color of Flat-Grained
Wood ovendrya Worst)b 4, Least) 2, Most) heartwood Surface
Softwood
Western redcedar .............. 0.32 I 1 1 Brown Distinct
Cypress ................................ 0.46 I 1 1 Light brown Strong
Redwood ............................ 0.40 I 1 1 Dark brown Distinct
Eastern white pine .............. 0.35 II 2 2 Cream Faint
Ponderosa pine .................. 0.42 III 2 2 Cream Distinct
White fir ................................ 0.39 III 2 2 White Faint
Western hemlock ............... 0.45 III 2 2 Pale brown Faint
Spruce ................................. 0.35 III 2 2 White Faint
Douglas fir ........................... 0.48 IV 2 3 Pale red Strong
Southern yellow pine ......... 0.51 IV 2 2 Light brown Strong
Hardwood
Eastern cottonwood .......... 0.40 III 4 2 White Faint
Magnolia ............................. 0.48 III 2 — Pale brown Faint
Yellow poplar ...................... 0.42 III 2 1 Pale brown Faint
Lauan (plywood) ................. —c IV 2 2 Brown Faint
Yellow birch ......................... 0.62 IV 4 2 Light brown Faint
Gum ..................................... 0.52 IV 4 2 Brown Faint
Sycamore ............................ 0.59 IV — — Pale brown Faint
American elm ..................... 0.50 V or III 4 2 Brown Distinct
White oak ............................ 0.64 V or IV 4 2 Brown Distinct
Northern red oak ................ 0.63 V or IV 4 2 Brown Distinct
(a) Specific gravity based on ovendry weight and volume.
(b) Woods ranked in group V are hardwoods with large pores, which require wood filler for durable painting. If all the pores are properly filled before painting, the wood
could be classified in group II.
(c) The specific gravity of different species varies from 0.33 to 0.55.
From the standpoint of finishing, The amount of water vapor that can vary from these average values. Even in
knotty grades of siding are better suited be absorbed depends primarily on the very humid areas, the RH is rarely high
for natural finishes such as water-repel- relative humidity (RH) of the surround- enough or long enough to bring the mois-
lent preservatives and semitransparent ing air. If wood is stored at 0% RH, the ture content of wood above 20%. Wood
stains than for paint systems. Paint ad- moisture content will eventually reach that is warmed by the sun experiences a
hesion is poor over dense knots, and 0%. If wood is stored at 100% RH, it will virtual RH far below the ambient RH.
resin bleeding of knots can discolor most eventually reach fiber saturation (about Wood will dry faster and become drier
paints unless they are sealed with shel- 30% water). Of course, if it is kept at a than expected given the ambient RH.
lac or a similar product prior to priming. constant RH between these two ex- This is why checking often occurs on
tremes, the wood will reach a moisture
content between 0 and 30%. The mois-
Moisture Content ture content is controlled by the RH,
and when the moisture content is in bal-
The moisture content of wood is the ance with the RH, the wood is at its
amount of water contained in the wood. equilibrium moisture content. This rarely
Moisture content includes both water happens because as the RH changes, so
absorbed into the wood cell wall and does the moisture content of the wood,
free water within the hollow center of and an atmospheric RH is almost al-
the cell (and within the vessels for hard- ways changing. It varies through daily
woods), and it is expressed as a weight and seasonal cycles, thus driving the
percentage. moisture content of wood through daily
and seasonal cycles. Equilibrium mois-
MC (%) = Mass (undried) – Mass (oven ture content of wood cannot be changed
dried) × 100 by the application of finishes. Finishes
Mass (oven dried) affect only the rate at which absorption
occurs. Finishes can decrease daily and
The amount of water that wood can ab- seasonal moisture absorption and des-
sorb (i.e., that can be bound in the cell orption, but they do not change the equi-
wall) depends on the wood species. Most librium moisture content.
species can absorb about 30% of their
Wood exposed outdoors cycles
weight in water. When the wood reaches
around a moisture content of about 12%
this limit to the amount of water that
in most areas of the United States. In the
can be bound in its wood cell walls, it is
Southeast, average moisture content can
at the fiber saturation point. Wood can
be slightly higher, and, in the South-
reach the fiber saturation point by ab- Figure 4—Formation of knots in
west, the average can be lower (9%).
sorbing either liquid water or water va- the stem.
Daily and annual moisture content will
por.
Vol. 72, No. 902, March 2000 37R.S. Williams et al.
by discoloration. The amount of extrac-
tives within the heartwood depends on
the age of the tree, thus the heartwood
of younger trees will naturally have a
lower extractive content than that from
older trees. Although lumber from
younger trees is less durable than lum-
ber from old growth trees, extractive
bleeding through finishes is less prob-
lematic. For clear grades of wood, ex-
tractive bleeding can be eliminated with
proper priming, good building design,
and conscientious construction practices.
(b) Water-Insoluble Extractives
Water-insoluble extractives, such as
pitch and resin, may also interfere with
Figure 5—(a) Loose knot in flat grained the appearance of a painted surface. In
board and (b) spike knot in vertical- some species, small amounts of pitch
grained board (southern pine). form in the wood. In other species, the
(a)
pitch can form in large deposits called
pitch pockets. If the wood is kiln-dried
at high temperature, the pitch can be
hardened or set in the wood. Specific
decking boards; the surface is much drier during its service life. The moisture con- kiln schedules have been developed for
than the rest of the board. The dryness tent and thus the dimensions of the piece many wood species to accomplish this.
causes shrinkage of the top of the board, will still fluctuate somewhat, depend- Unfortunately, some of these schedules
which goes beyond the elastic limit of ing on the cyclic changes in atmospheric can discolor the surface of the wood. If
the wood at the surface, and checks form RH, but the dimensional change will not pitch is not set, it can become fluid
parallel to the grain. be excessive. Therefore, film-forming fin- enough during periods of warm weather
ishes (such as paints) will not be stressed to flow to the surface of the wood. If the
As mentioned, fiber saturation is the
unnecessarily, and service life should be wood has been painted, the pitch tends
greatest amount of water that can be
better. to soften and discolor the paint. Young
absorbed by wood via water vapor ab-
sorption. This absorption is rather slow Plywood, particleboard, hardboard, growth knotty siding products that have
compared with the moisture changes and other wood composites undergo a been air-dried rather than kiln-dried may
that can occur through absorption of liq- significant change in moisture content be more prone to pitch bleeding. Young
uid water. Liquid water can quickly during manufacture. Frequently, the growth products will typically have
cause the wood to reach fiber satura- moisture content of these materials is smaller pitch pockets than those found
tion, and it is the only way to bring the not known and may vary depending on in old growth. Additionally, knots of
moisture content of wood above fiber the manufacturing process. To improve many softwood species contain an abun-
saturation. As wood continues to absorb the service life of the finish, wood com- dance of resin that can sometimes cause
water above its fiber saturation point, posites should also be conditioned prior paint to turn yellow-brown over the
the water is stored in the hollow center to finishing. knots. Primers formulated to block wa-
of the wood cell; when all the air in the
hollow center has been replaced by wa-
ter, the wood is waterlogged and mois- Water-Soluble Extractives
ture content can be as high as 200%. The
In addition to the physical structure
sources and ways by which wood can
and moisture content of wood, wood
get wet sometimes seem endless. The
chemical composition affects the perfor-
result is always the same — poor perfor-
mance of paints and finishes. Old growth
mance, both of the wood and the finish.
timber is characterized by a high per-
Water also causes peeling of paint. centage of heartwood. Heartwood con-
Even if other factors are involved, water tains extraneous compounds such as
accelerates paint degradation. If the tannins and other polyphenolics. These
moisture content of the wood exceeds compounds, called extractives, impart
20% when the wood is painted, the risk natural decay and insect resistance to
of blistering and peeling is increased. heartwood of several species of wood,
Moreover, dark water-soluble extractives such as redwood and the cedars. The
in woods like redwood and western presence of water-soluble extractives can
redcedar may discolor paint shortly af- discolor the surface of paints and fin-
ter it is applied. Fortunately, the mois- ishes and is often referred to as extrac-
ture content of lumber can be controlled. tive bleeding.15 Although these extrac-
But all too often this critical factor is tives can cause problems with discol-
neglected during the construction and oration of finishes if not sealed with a
finishing processes. It is best to paint stain blocking primer, the decay resis-
Figure 6—Microscopic view of
wood when its average moisture con- tance that some of them impart to wood
softwood with a resin duct.
tent is about that expected to prevail greatly outweighs disadvantages caused
38 Journal of Coatings TechnologyWood Properties Affecting Finish Service Life
ter-soluble extractives will not block Lignin comprises 20 to 30% of the sanding provides a much better surface
these resins. wood surface. It is a polymeric substance for painting.
The resins in some softwoods can oc- that glues the celluloses together. Be-
cur in small ducts (Figure 6). Depending cause lignin is affected by photodegra-
dation more than are celluloses, lignin Juvenile Wood
on the grain orientation, a large number
of these ducts can intersect the surface degrades and cellulose fibers remain The presence of juvenile wood, which
of the lumber. Paint on these surfaces loosely attached to the wood surface. is often characterized by lower density
may be more prone to discolor. If vari- Further weathering causes fibers to be and higher longitudinal shrinkage, is re-
ous cuts of lumber (i.e., grain orienta- lost from the surface (a process called sponsible for much of the decrease in
tion) are used to make finger-jointed erosion), but this process is so slow that strength of young growth timber. Juve-
moldings, the discoloration of the vari- on average only about 6 mm (1/4 in.) of nile wood is produced during the early
ous pieces of wood in a particular piece wood is lost in a century (Figure 7). This growth and development stages of trees;
of molding can cause a checkerboard erosion rate is slower for most hard- therefore, it is located around the pith of
pattern of discoloration. This can also woods. the tree. Much of the warp, crook, and
occur with water-soluble extractives, Biological attack of a wood surface bow in lumber containing the pith or
particularly with finger-jointed western by microorganisms is recognized as a sawn close to the pith is caused by the
redcedar or redwood. contributing factor to color change or presence of juvenile wood. Cross-
graying of wood. This biological attack, grained cracking of lumber is often a
commonly called mildew, does not cause sign of juvenile wood (Figure 8). Because
Weathering erosion of the surface, but it may cause of the younger age and smaller diam-
initial graying or an unsightly dark gray eter of trees harvested from young
Weathering is the general term used and blotchy appearance. The microor- growth forests, a higher proportion of
to describe the degradation of materials ganisms primarily responsible for gray the volume in a log consists of juvenile
exposed outdoors. This degradation oc- discoloration of wood are commonly wood. Juvenile wood is particularly
curs on the surface of all organic materi- found on weathered wood. problematic in the short rotation (har-
als, including wood, as well as finishes vested at 20 to 30 years), plantation
used on wood, such as paints and stains. grown softwoods.
The process occurs through photooxi- Paint Adhesion to
dation of the surface catalyzed by ultra- Weathered Wood
violet (UV) radiation in sunlight, and it Manufacturing
is augmented by other processes such as Although the erosion of the wood
washing by rain, changes in tempera- surface through weathering is a slow Characteristics
ture, abrasion by windblown particles, process, the chemical changes that occur
and changes in moisture content. Al- within a few weeks of outdoor exposure Grain Orientation
though the weathering process can take can drastically decrease the adhesion of
Several properties of flat-grained lum-
many forms depending on the exposed paints subsequently applied to the
ber differ from vertical-grained lumber.
material, in general, the process begins weathered surface. It is fairly obvious
with a color change, followed by slow that a badly weathered, powdery wood
erosion (loss of material) from the sur- surface cannot hold paint very well. This
face. The surface initially develops slight fact is not so obvious for wood that has
checking. With some materials, deep weathered for only two to three weeks.
cracks may ultimately develop. Weath- The wood appears sound and much the
ering is dependent on the chemical same as unexposed wood. However,
makeup of the affected material. Because when smooth-planed boards that had
the surface of a material may be com- been preweathered for 1, 2, 4, 8, or 16
posed of many different chemicals, not weeks were painted, the paint had a dras-
all materials on the surface may erode at tic loss in adhesive strength after four
the same rate. weeks of preweathering.16 Similar results
The surface of wood consists of four are reported in other publications.17–28
types of organic materials: cellulose, In subsequent studies using similarly
hemicellulose, lignin, and extractives. preweathered boards, the time until the
Each of these materials is affected by paint started to peel was directly related
the weathering process in a different to the preweathering time.29 Additional
way. The extractives undergo changes weathering of these panels showed that,
upon exposure to sunlight and lighten for panels preweathered 16 weeks, the
or darken in color. With some wood paint started to peel within three years.
species, this color change can take For panels preweathered for only one
place within minutes of exposure. week, the paint started to peel after 13
Changes in the color of the surface are years (Williams, unpublished data). Pan-
accompanied by other changes that els that were not preweathered showed
affect the wettability and surface no sign of peeling after 13 years. The
chemistry of the wood. The mecha- paint system was a commercial oil-alkyd
nism of these early changes is not very primer with two acrylic latex topcoats
over planed all-heartwood vertical- Figure 7—Western cedar exposed
well understood, but these changes to weather for eight years. Top,
can have a drastic effect on the sur- grained western redcedar.
weathered specimen with top half
face chemistry of wood and thus the The best remedy for restoring a protected by a metal strip. Bottom,
wood’s interaction with other chemi- weathered wood surface is to sand it side view micrograph of weathered
cals such as paint and other finishes with 50 to 80 grit sandpaper. Even if the surface.
and adhesives. wood has not been weathered, scuff
Vol. 72, No. 902, March 2000 39R.S. Williams et al.
manufacturing practices, closer attention contaminants, but it also roughens the
to machine set up and more frequent surface slightly and removes raised
sharpening of knives and greater atten- grain. The abrasion of the surface is par-
tion to quality control procedures are ticularly important on the latewood por-
required when surfacing flat-grained tion of the growth rings where the wood
lumber. is more dense, and it is more difficult to
attain finish adhesion. Sanding helps
prepare this portion of the wood. Under
Surface Texture magnification, sanded surfaces emulate
Surface quality and texture play an characteristics similar to saw-textured
important role in finish performance, and surfaces but to a lesser degree. More
the variables that influence the surface surface area is exposed in the sanding
characteristics of the wood are numer- process, creating better conditions for
ous. Rough sawn, sanded, saw-textured, finish adhesion.
and smooth-planed surfaces all influence
finishes differently.
Moisture Content and Drying
Generally speaking, rough sawn and
saw-textured surfaces require more fin- Significant regional geographic dif-
Figure 8—Warping (top) and ex- ish than smooth-planed surfaces, and ferences exist in lumber drying practices
cessive surface degradation (bot- they provide longer lasting performance resulting from species mix, building
tom) resulting from juvenile wood. because more finish must be applied. practices, and climate throughout North
Rougher surfaces provide more tooth for America. Wood products are available
finish adhesion and actually have much green (unseasoned), S-Dry (Surfaced-
more surface area. Rough surfaces also Dry, 19% or less moisture content), or
These include appearance or figure, di- have far less of a problem with grain kiln-dried (typically 10 to 15% moisture
mensional stability, potential for grain raising than does flat-grained lumber content). Only dimension and finish lum-
raising and grain separation, and ability because of less grain compression caused ber two inches and thinner in nominal
to hold film-forming finishes. The broad by planing processes. As the timber re- thickness (standard 38 mm) may be
alternating bands of earlywood and late- source changes from an old growth to a available S-Dry or kiln-dried. Larger di-
wood that make up the annual growth young growth supply, the ratio of flat mensions of solid sawn lumber should
rings are highly visible on the surface of grain to vertical grain is increasing dra- be assumed to be green. Higher grades
flat-grained lumber. This grain figure is matically. For this reason, rough sawn of lumber such as those used for archi-
valued for applications such as indoor and saw-textured surfaces are being used tectural finish are normally kiln-dried.
paneling where maximum natural grain more frequently in an effort to decrease
pattern of the wood is desired. As previ- grain raising and improve finish perfor-
ously mentioned, in flat-grained lum- mance on flat-grained wood products.
ber, knots will appear round or oval The bark side of flat-grained prod-
rather than spiked as in vertical-grained ucts weathers differently than the pith
lumber. Since tangential shrinkage is side; therefore, lumber patterns that have
about twice the rate of radial shrinkage, an obvious exposed face can be manu-
flat-grained lumber can be expected to factured to leave the bark side exposed.
shrink across its face much more than This is referred to as “graining the piece.”
vertical-grained lumber. Some patterns are reversible and in these
Perhaps the greatest difficulty asso- cases it is up to the installer to grain the
ciated with the use of flat-grained lum- piece and apply the product with the
ber is the increased potential for grain bark side exposed. In cases where the
raising and grain separation (Figures 9a customer knows which face of a pattern
and 9b). Raised grain, which results in a will be exposed to weathering, the manu-
corrugated appearance on the surface of facturer should be notified prior to
flat-grained lumber, usually occurs when manufacturing so that the pattern can
the harder latewood portion of each an- be grained and manufactured properly.
nual growth ring is projected above the Graining solid sawn lumber is a com-
level of the softer earlywood. This usu- mon practice that can be done by an
ally occurs when dry flat-grained lum- experienced planer or molder operator.
ber is allowed to pick up moisture. When This practice, however, is not always
the interface between the latewood and practical when manufacturing finger-
earlywood becomes loosened, the result jointed lumber, and the chances of en-
is separated grain, also known as loos- countering grain raising and the associ-
ened grain, shelling, feathering, or lift- ated finish problems may be higher for
ing of the grain. Raised or separated these products. For this reason, the use Figure 9—(a) Paint failure caused
grain is much more pronounced on the of saw-textured patterns is especially by raised grain; (b) Raised grain on
pith side than on the bark side of flat- important with finger-jointed wood sub- book-matched specimens of planed
grained lumber. The primary method of strates. lumber. The cross sections of a
preventing problems with grain separa- Recent studies have shown that sand- planed board illustrate grain raise
tion is to orient the bark side rather than ing surfaces may improve finish perfor- caused by water. Top, surface im-
the pith side of the product to the mance and durability. Sanding with 50 mediately following planing but not
weather. Since grain raising and grain to 80 grit sandpaper not only cleans the wetted; bottom, the same planed
separation can be exacerbated by poor surface by removing dirt, oil, and other surface but wetted.
40 Journal of Coatings TechnologyWood Properties Affecting Finish Service Life
Common grades of lumber are usually or other film-forming finishes. Green structures throughout the East Coast of
available S-Dry and are less frequently lumber can be used in situations where the United States.
kiln-dried. penetrating stains and finishes will be Wood buildings that are properly de-
applied, but the wood surface must first signed to shed exterior water and to
Wood moisture content is one of the be allowed to dry sufficiently to accept
most critical factors governing the per- avoid trapping moisture from interior
the finish. This can usually be accom- sources routinely give service lives of
formance of paints and other film-form- plished during a few days of dry
ing finishes. Typically, paints and other much longer than 100 years. This is par-
weather. As described previously, ticularly true if the wood is maintained
surface coatings cannot be successfully longer exposure times lead to surface
applied to wood that has a surface mois- with a paint finish. The roof of the build-
degradation and should be avoided. ing protects the wood from most of the
ture content in excess of approximately Other problems associated with the use
20%. If the moisture content is too high, water. The finish stops the weathering
of green lumber products include the of the wood’s surface because the pig-
continued movement of this moisture natural shrinkage that occurs as these
out of the wood may result in extractive ments in the paint completely block the
products acclimate to ambient condi- UV radiation in sunlight, which cata-
bleeding and blistering and peeling of tions. Checks, splits, and warping may
film-forming finishes. For best results lyzes the degradation of the wood. The
occur from rapid, uneven shrinkage of paint itself undergoes UV degradation,
the wood should be at a moisture con- lumber. These characteristics can be
tent typical of what it will have during and a paint system comprised of a primer
minimized by using higher grades of and two topcoats lasts about 10 years in
its service. For most areas of the United lumber.
States and Canada, this is about 12%. full sunlight. During this time, the top-
Penetrating finishes such as water-re- coats erode and when the primer begins
pellent preservatives and semitranspar- to show, the surface should be repainted.
ent stains may be applied to products Performance Expectations By keeping an intact paint film on wood,
that have a dry surface but which still the wood’s surface can be protected in-
may contain a high enough internal or Unlike building materials such as definitely.
core moisture content to be considered steel and masonry, wood is a biological Wood used in exterior landscaping
green. Since penetrating finishes do not product. As such, individual pieces vary projects such as decks and fences is gen-
block moisture movement, they allow more in appearance, properties, and per- erally considered to have a much shorter
the wood to dry but do not blister or formance. The life expectancy of a lum- expected service life. There are a num-
peel. ber product is related to a number of ber of reasons for this. Decks and fences
direct and indirect factors. Direct factors are often in direct ground contact or in
Lumber manufacturing can also have include the properties of a particular spe-
an effect on the overall performance of a very close proximity to the ground. This
cies, the characteristics of the grade, and results in a higher risk for termite and
coating on wood. As previously dis- the quality of manufacture. Indirect fac-
cussed, proper drying plays an impor- wood decay fungus infestations. The
tors include how the product is used horizontal exposure of decks results in a
tant role in producing a wood product after it has left the manufacturing facil-
that will be dimensionally stable and much higher degree of weathering from
ity. These indirect factors include han- UV radiation, precipitation, and other
have a durable finish. Proper kiln dry- dling, storage, installation, and mainte-
ing provides the best situation for wood elements. Decks are also directly exposed
nance. Siding is but a single component to the wear from foot traffic. For preser-
because it creates a balanced piece of of a building. Its performance depends
lumber that has an even moisture con- vative-treated wood, the service life is
on many critical factors including cli- often determined by the weathering of
tent throughout the piece. Kiln-dried mate, the design of the structure, the
lumber has less tendency to shrink or the decking boards. The NAHB estimates
skill of the person doing the installation, the life expectancy of wood decks and
warp, but the most important character- and the interaction with other materials
istic from the standpoint of finishing is fences to be 15 years and 12 years, re-
such as fasteners, flashing, sheathing,
that kiln-dried lumber develops less sur- spectively.30 We consider these life ex-
and house-wrap or felt.
face checking. Paint applications are es- pectancies to be conservative. There are
pecially susceptible to performance fail- Under ideal conditions, wood can many examples of decks and fences last-
ures when surface checking of the wood have an almost indefinite service life. As ing much longer when properly de-
substrate occurs. These checks initiate a rule of thumb, most low-rise residen- signed, constructed, and maintained. We
cracking and peeling of the coating. Kiln- tial structures are designed and built believe that a reasonable expectation of
drying dramatically decreases this con- with an intended service life of 100 years. service life for these and other types of
dition. It is assumed that after 50 years, the wood landscaping structures is 20 to 25
structure will require extensive renova- years.
The finish applied to air-seasoned or tion that will include the replacement of
S-Dry lumber is more durable than that the exterior cladding. A service life for
applied to green lumber. However, these exterior wood siding on the order of 50
partially dried products will probably to 100 years is therefore generally con- Summary and Conclusions
not give the same quality of finish per- sidered acceptable. The National Asso-
formance as kiln-dried lumber. While ciation of Home Builders (NAHB)30 gives Of the vast array of wood properties
some of the moisture has been removed a life expectancy for wood siding of only that affect the performance of finishes
in S-Dry and air-seasoned lumber, a per- 10 years if constantly moistened but up for wood, several stand out as having a
centage of excess moisture is still present to 100 years if properly maintained. major effect on the long-term service life
in the wood and this can be detrimental Keeping wood and wood products dry of finishes. The growth rate and the grain
to film-forming finishes such as paint. is essential for long-term service life. A pattern that results from the growth rate,
Partially dried products offer dried sur- service life of 100 years is easily obtained particularly the relative width and den-
faces that are satisfactory for applying if the wood is kept dry and maintained sities of the earlywood and latewood,
penetrating finishes such as water repel- with a finish. There are many examples affect the dimensional stability and thus
lents and semitransparent stains. of wood siding lasting more than 200 the amount of stress at the wood–coat-
Green lumber is generally not suit- years; for example, the siding on Mount ing interface. Coating systems lacking
able for immediate finishing with paint Vernon and numerous other historic sufficient flexibility can crack as a result
Vol. 72, No. 902, March 2000 41R.S. Williams et al.
of these stresses. The grain orientation Material,” Gen. Tech. Rep. FPL–GTR– (20) Evans, P.D., Thay, P.D., and Schmalzl,
of lumber is affected by the way it is cut 113, USDA Forest Service, Forest Prod- K.J., “Degradation of Wood Surfaces
from a log. Flat-grained surfaces are ucts Laboratory, Madison, WI, 1999. During Natural Weathering. Effects on
much more difficult to finish than are (9) Williams, R.S., Knaebe, M.T., and Feist, Lignin and Cellulose and on the Adhe-
W.C., Finishes for Exterior Wood, Forest sion of Acrylic Primers,” Wood Sci.
vertical-grained surfaces. The high den- Products Society, Madison, WI, 1996. Technol., 30, No. 6, 411 (1996).
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rather poor paint adhesion properties. Federation Series on Coatings Technology, K., “Wood Protection—The Interaction
The extremely large amount of longitu- Federation of Societies for Coatings Between Substrate and the Influence on
dinal dimensional changes of juvenile Technology, Blue Bell, PA, 1996. Durability,” J. Oil & Colour Chemists’
wood can often cause cross-grained (11) Browne, F.L., “Wood Properties that Af- Assoc., 66, No. 11, 345 (1983).
cracking of wood with resulting crack- fect Paint Performance,” FPL Report (22) Miller, E.R., “Chemical Aspects of Ex-
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suitable sealer to avoid discoloration. USDA Forest Service, Report FS-540, (23) Bravery, A.F. and Miller, E.R., “The Role
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Through understanding the proper- Williams, S., “Changing Nature of Wood Information Sheet 20/77, Building Re-
ties of wood and using finish systems Products–What Does it Mean for Coat- search Establishment, Princes Risbor-
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designed and well-constructed build- OF COATINGS TECHNOLOGY, 71, No. 890, 61 gland, 1977.
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