United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL 366

February, 1980

Durability of Exterior

Natural Wood Finishes

in the Pacific Northwest
ABSTRACT Note:

There is a growing demand for

natural exterior wood finishes that

retain the original attractive appearance

of wood with the least change in color

and masking of grain. A number of

experimental finishes are being

evaluated for their performance in the

cool, moist climate of Olympia, Wash.

This study, started in 1966, has included

observations on the weathering

performance of 48 experimental

finishes, both transparent and

pigmented. Of these, 36 are still on

exposure and inspected annually.

Generally, water-repellent preservatives

are the least durable of the natural

finishes. Pentachlorophenol was the

most effective mildewcide used in these

finishes. Semitransparent, penetrating

stains provide greater durability as

compared to the transparent waterrepellent preservatives. Stains

containing only 20 percent binder

provided excellent protection even after

8 years’ exposure. Latex stains

containing copper chromate were

excellent durable natural finishes

Simple water solutions of chromiumcontaining chemicals acted as durable

“natural” finishes and were especially

effective after one refinishing. These

exposure results indicate that natural

finishes for wood can be used

successfully in climates where mildew

growth is a problem and service lives of

at least 6 years could be expected.

Trade and company names are

included for the benefit of the reader

and do not imply any endorsement or

preferential treatment of the product by

the US. Department of Agriculture.

This publication reports research

involving pesticides It does not contain

recommendations for their use, nor

does it imply that the uses discussed

here have been registered. All uses of

pesticides must be registered by

appropriate State and Federal Agencies

or by both before they can be

recommended.

CAUTION: Pesticides can be

injurious to humans, domestic animals,

desirable plants, and fish or other

wildlife-if they are not handled or

applied properly. Use all pesticides

selectively and carefully. Follow

recommended practices for the

disposal of surplus pesticides and

pesticide containers.

Acknowledgment

The authors thank Jill M.

Wennesheimer and Peter G. Sotos for

valuable laboratory assistance.
United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory1

Research

Paper

FPL 366

1Maintained at Madison, Wis., in cooperation

with the University of Wisconsin.

2

Italicized numbers in parenthesis refer to

literature cited at end of report.

February, 1980

Durability of Exterior

Natural Wood Finishes

in the Pacific Northwest

By

WILLIAM C. FEIST, Chemist

EDWARD A. MRAZ, Technician

NATURAL WOOD

FINISHES

Penetrating

In many locations throughout the

United States, there is a continuing and

growing trend toward the use of more

natural colors and finishes to protect

wood siding on the exterior of

structures. Architects, builders, and

owners are increasingly interested in

the “natural look” for their homes,

apartments, churches, and commercial

buildings. A truly successful and ideal

natural exterior wood finish is one

which will retain the original, attractive

appearance of wood siding with the

least change in color and the least

masking of wood grain and surface

texture. The most natural appearance

for wood would be achieved without a

protective finish. Unfortunately,

unprotected wood exposed outdoors is

soon changed in appearance by the

adverse effects of light, moisture, and

the growth of micro-organisms on the

surface (4,9).2

In outdoor weathering of smooth

wood, original surfaces become rough

as grain raises and the wood checks,

and the checks sometimes grow into

large cracks; grain may loosen, boards

may cup and warp. and pull away from

fasteners. The roughened surface

changes color rapidly, gathers dirt and

often mildews, and may become

unsightly; the wood loses its surface

coherence and becomes friable. Where

salt in the atmosphere may inhibit

excessive mildew growth, natural

weathering may create a changed but

desirable silvery appearance to the

exposed wood. In dry climates (or cold),

a rustic. brown-to-gray patina may

result. In many humid locations,

however, weathering is often

accompanied by a surface growth of

dark gray, blotchy mildew, which may

remain unsightly until the wood has

weathered for many years.

An ideal natural finish, therefore,

should inhibit the growth of mildew

micro-organisms, protect against

moisture and sunlight, and not change

surface appearance or color of the

wood.

Types

Natural wood finishes fall into two

broad classifications-film-forming and

penetrating.

Film-forming

Varnishes are the primary transparent

film-forming materials used for natural

wood finishes, and their use greatly

enhances the natural beauty and figure

of wood. They lack exterior permanence

unless protected from direct exposure

to sunlight, and varnish finishes on

wood exposed outdoors will generally

require refinishing every 1 to 2 years.

The penetrating finishes are the

second broad classification of natural

wood finishes. These finishes do not

form a film on the wood surface and are

further divided into (1) transparent or

clear systems and, (2) pigmented or

semitransparent systems.

Transparent.– Water-repellent

preservatives (WRP) (5) are the most

important of the transparent penetrating

systems. It has been found that treating

wood surfaces with WRP will protect

wood exposed outdoors with little

change in appearance. A clean, goldentan color can be achieved with most

wood species. The treatment reduces

warping and cracking, and prevents

water staining at edges and ends of

wood siding, and helps to control

mildew growth. The first application of

WRP may protect exposed wood

surfaces for only 1 to 2 years, but

subsequent reapplications may last 2 to

4 years. depending on exposure.

Semitransparent.– The

semitransparent stains (2) are the

second of the penetrating natural wood

finishes. These stain finishes provide a

less natural appearance because they

contain pigment and mask the original

grain and color of the wood. They are
generally much more durable then are

varnishes or WRP’s and provide more

protection against weathering. With

these stain systems, weathering is

slowed by retarding the alternate

wetting and drying of wood end the

presence of pigments on the wood

surface minimizes the degrading effects

of sunlight. The amount of pigment in

the semitransparent steins varies

considerably, and different degrees of

protection against ultraviolet

degradation end masking of the

appearance of the original wood

surface can be achieved.

Latex stains are also described as

semitransparent. These pigmented

“natural” finishes are generally

nonpenetrating but do retain the

surface texture of the wood. Several

pigmented semitransparent latex steins

are included in this study.

Waterborne salts.–Waterborne

inorganic salts (3) are a special group of

penetrating finishes. These surface

treatments which result in a natural

finish are related to the semitransparent

penetrating finishes, since they change

the color of the wood end leave a

surface deposit of material similar to the

pigment found in the semitransparent

steins.

Opaque Stains

Solid-color or opaque steins are

another classification of finishes

sometimes described as natural wood

finishes. These finishes are high in

pigment content end completely mask

the color and figure of the wood.

Surface texture is retained end these

generally nonpenetrating finishes yield

wood against ultraviolet degradation,

a flat appearance. They do protect

but tend to perform more like paints in

that they do not penetrate the wood

surface to any degree. The remainder of

this publication will be concerned with

the performance of the penetrating

finish systems, both clear end

semitransparent. No opaque or solidcolor systems were investigated.

Advantages of

Penetrating Stains

climate. Such climate often results in

excessive mildew growth end special

used to study the performance of

challenge to the performance of natural

finishes. Previous publications in this

natural finishes in that cool, moist

study initiated in 1966 describe

exposure results up to 1975 (7,8). This

report summarizes exposure results of a

number of finishes applied in 1974 as

well as continuing exposure results on

finishes applied in 1966, 1970, 1972,

and 1973, some of which were

subsequently refinished.

Penetrating exterior wood finishes

offer many advantages for protecting

exposed wood. They (1) provide a

natural appearance; (2) do not peel or

blister since they do not form a surface

film or coating; (3) are especially

durable and well suited for roughtextured surfaces, severely exposed

horizontal wood surfaces (decks,

railings, end steps), and perform well on

knotty, flat-grained surfaces; (4) often

contain water repellents which inhibit

the rapid pickup of rein end dew; (5) do

not trap moisture in wood which may

encourage decay; end (6) are easily

applied and renewed.

FPL Research

The Forest Products Laboratory (FPL)

has a long history of research on

finishes end surface treatments that

enable wood to provide greater

serviceability end more user satisfaction

(23.6). An outdoor exposure site in the

Pacific Northwest at Olympia, Wash.. is

The objective of this research is to

evaluate the weathering performance of

“natural” exterior finishes that protect

the wood surface from the deteriorating

effects of sunlight, moisture, and the

growth of mildew. Systems included are

(a) water solutions of inorganic salts (3 )

that absorb ultraviolet light and protect

surface wood fibers from

photodegradation, (b) pigmented oil or

latex stains which retard the penetration

of ultraviolet light, and(c) transparent

WRP finishes (5). These lest are

included because of their ability to

retard mildew growth and control water

penetration.

TEST PROCEDURES,

MATERIALS, METHODS

Exposure Panels

Three species of wood siding with

both rough end smooth surfaces.

installed vertically end horizontally on a

test panel, were exposed on a test fence

et 90° facing south at Olympia, Wash.

Table 1.–Water-repellent preservative formulation

Composition (weight percent)

1Mautz Paint Company exterior Polyurethane, 50 percent solids.

2Dow Sylgard 184.

3Metasol 57 (Phenyl mercury propionate).

4Bis (tri-n-butyltin) oxide.

5Metallic zinc or colbalt as naphthenate.

6Solvents, primarily mineral spirits.

2
Table 2.–Semitransparent penetrating stain and latex stain formulations

Composition (weight percent)

1Mautz Exterior, V 103, 50 percent solids.

2Cargill 1210, 50 percent solids.

3Rohm and Haas AC-388.

4Elmer's Glue-All.

5Universal colors, 50 percent solids. 6Solvents, primarily mineral spirits.

7Determined by dividing the volume of pigments alone by the combined volume of resin plus pigments and multiplying by 100.

All specimens with a given finish were

grouped in one composite panel on the

test fence as shown in figure 1.

Construction details were described

earlier (8). The three wood species used

were Douglas-fir plywood

(Pseudotsuga menziesii (Mirb.) Franco);

redwood (Sequoia sempervirens (D.

Don) Endl.); and western redcedar

(Thuja plicata Donn).

Wood Finishes

All natural finishes included in this

study were prepared at the Forest

Products Laboratory. Three general

types were included: (1) WRP’S; (2)

stains, both semitransparent

penetrating (solventborne) and latex

(waterborne); and (3) water solutions of

inorganic salts. Compositions of the

finishes evaluated are shown in tables 1

through 3. Each finish is identified by a

number for convenience. Subsequent

reference to any finish will be by the use

of its identification number.

Identification numbers are identical to

those used in the last report on this

subject (7). Numbers with an “a” suffix

represent finishes which replaced those

described earlier (7.8). These “a”

finishes were applied over new panels

of the three wood species. All finishes

were applied by brush, using spreading

rates of 200 to 250 square feet per

gallon.

1

Finish 16-1 applied, allowed to air dry, and finish 16-2 then applied.

2Clear finish consisting of 2 coats of acrylic resin (62 g) and linseed oil (33 g) applied after treatment with

inorganic salt.

3Sodium salt of pentachlorophenol.

Several finishes investigated included

surface pretreatment and finishing.

Finish No. 30 was a combined

treatment. The panel was first treated

Table 3.–Waterborne inorganic salt composition

Finish number

with a water solution containing copper

sulfate and sodium dichromate.

(Referred to for convenience as copper

chromate.) After a 24-hour drying

period, the surface-treated panel was

finished by brush application of two

coats of an aqueous suspension

containing acrylic latex resin (Rohm

and Haas Co., AC-388, 62 grams) and

boiled linseed oil (33 g) (48 hours of

drying between coats). A wood surface

pretreatment was also included in the

latex stain finish evaluations (7a to 12a).

One-half of each portion of the vertical

and horizontal boards on the test panels

was brush-treated with a water solution

of copper chromate (9.9 g of CuSO4 • 5

H2O, 5.9 g of Na2Cr2O7 • 2 H2O, and 84.2

g of H2O). The treated surface was

allowed to dry for 48 hours and the latex

stain then applied.

Evaluation of Finishes

All test panels are inspected annually.

The WRP-finished panels were

evaluated by assigning a rating to each

species on the panel based on the

amount of graying (mildew growth) and

surface roughening observed. Separate

values were assigned to rough and

smooth boards. The rating system used

was based on a descending scale from

10 to 0; 10 represents no graying and 0

represents a totally grayed panel with

heavy mildew growth. A photographic

representation of each of the values

from 10 to 0 was used for reference

similar to those used for the American

Society for Testing and Materials

(ASTM) tests for chalking (D 659-74).

checking (D 660-44), cracking (D 661-

3
Figure 1.–Typical test panel on exposure fence. Left one-third, Douglas-fir

plywood; center one-third, redwood; right one-third, western redcedar. (Panel

dimension: 4 feet 6 inches high; 8 feet wide.)

(M 147 122-20)

Table 4.–Finish performance ratings

1Similar values assigned to samples to evaluate erosion were based on photographic representations for each

value (1).

44), erosion (D 662-44), blistering (D compared with other panels. Excellent

714-56). and flaking (D 772-47) (1). The to poor overall ratings were assigned for

mildew-growth rating values for rough/ convenience in discussing results.

smooth boards and for each of the three Evaluations since the start of the study

species were totalled end values are shown in tables 5 and 6.

compared between panels. For

convenience, these combined 10 to 0

scale ratings were converted to an

overall general appearance scale of

excellent, very good, good, fair, end

RESULTS AND

DISCUSSION

poor (table 4). Semitransparent stains,

latex steins, and inorganic-salt finishes Water-Repellent

were evaluated by the degree of erosion Preservatives

of the finish (weathering) es well es by

surface mildew growth. As with the Water-repellent preservatives are

WRP, values from 10 to 0 were unpigmented and rely on a preservative

assigned, depending on panel or mildewcide to prevent growth of

condition. Values were combined end mildew and subsequent graying of

4

wood. The water repellent (often

paraffin wax) in the finish reduces

penetration of water, particularly into

the end grain, minimizing staining end

reducing decay (5). The WRP usually

contains a small amount of resin or

binder (drying oil) which helps to hold

the fungicide and wax in the wood and

retards finish degradation on the wood

surface. All WRP’s currently in test were

formulated using 0.5 percent paraffin

wax and mineral spirits as solvent (table

1).

Finish NO. 2 contains 5 percent

pentachlorophenol and 10 percent

boiled linseed oil. This formulation was

used to represent a typical WRP (5) for

comparison with other WRP’s

evaluated. As seen in table 5, the overall

perfomance of this WRP is judged fair

in resistance to mildew growth after 12

years of exposure end three

refinishings. The test panel has retained

a somewhat uneven brown appearance

even though 4 years have elapsed since

the last refinishing (table 6).

Finish No. 4. formulated with zinc and

cobalt naphthenetes, was reasonably

effective in performance. A fair

performance rating has been

maintained in the 4 years since the third

refinishing. This simple treatment

results in a soft, bleached appearance

versus the uneven brown of the

standard WRP, No. 2. Addition of 5

percent pentechlorophenol to the zinc

and cobalt naphthenate finish (No. 17)

greatly improved mildew resistance and

resulted in a panel with attractive brown

color even with 4 years of exposure

since the last refinishing (table 6). Other

modifications of the zinc and cobalt

naphthenete-containing finishes, No. 3a

with added exterior varnish and No. 19a

with both exterior varnish end 5 percent

pentachlorophenol, have been exposed

for 6 years. The improved performance

of the zinc and cobalt naphthenete with

added pentechlorophenol indicates that

this letter mildewcide is probably

necessary for effective resistance to

mildew growth end graying in the

Pacific Northwest.

A WRP containing 8 percent

pentachlorophenol (No. 37) performed

similar to No. 2, which contained 5

percent. This WRP finish was more

effective then finish No. 2 in preventing

graying and after 4 years of exposure

and no refinishing, it still rated fair in

overall appearance and has a somewhat

uneven brown color with about 30

percent graying. Replacement of part of

the varnish resin with a silicone (No.

13a) resulted in improved performance
compared to No. 37; a more even, very

soft brown appearance resulted with

about 20 percent graying apparent. The

silicone addition probably resulted in

greater water repellency and reduced

mildew growth.

Light additions of mercury-containing

mildewcide (0.5 pct phenyl mercury

propionate by weight) were used in

combination with 9.5 percent

pentachlorophenol (Nos. 18, 20). Both

finishes, using boiled linseed oil or

exterior varnish as binder, are rated es

very good 4 years after the last

refinishing. Test panels have a slightly

uneven soft brown appearance. No

additional experiments are planned with

mercury-containing mildewcides since

these materials are no longer permitted

in solvent-based finishes.

The remaining four WRP’s under

evaluation contained 2 percent bis (trin-butyltin) oxide (TBTO). Performance

after 1 year of exposure was very good

but mildew growth became quite rapid

after that (finish Nos. 25a, 31, 35, 36).

Mildew growth after 3 years of exposure

was very heavy for all but finish No. 31

which contained a silicone. All four

finishes were rated poor after 4 years of

exposure. While the TBTO-containing

WRP’s were effective after 1 year of

exposure, mildew growth became very

rapid thereafter and the test panels

were much darker and more uneven

than untreated controls of the panels

with finish No. 2, which contained 5

percent pentachlorophenol.

The best of the WRP’s evaluatedNos. 17, 18, 19a, and 20–maintained a

clean, relatively natural appearance,

free of mildew growth, on all boards in

the test panels for 4 years after

refinishing. Performance was best on

Douglas-fir. followed closely by western

redcedar and finally by redwood (table

6). The pentachlorophenol-containing

finishes (Nos. 18 and 20) gave all three

test species a slight golden-brown cast;

addition of zinc and cobalt naphthenate

to the pentachlorophenol gave a light,

bleached effect. Generally, several

years of weathering were required to

achieve the fully developed patina of the

WRP-type natural finish.

Semitransparent Penetrating

Stains

The performance of the proven FPL

natural finish (2) on exposure in

Olympia, Wash., was described earlier

(7). This high-solids stain (linseed oil)

had excellent ratings after 7 years of

exposure. Four finishes currently being

Table 5.–Condition of test panels1

Finish Years of exposure

evaluated (Nos. 21 to 24) are variations

of the FPL natural finish. These were

formulated with reduced linseed oil

content as well as replacement binders

to evaluate if the changes would

enhance performance and reduce

mildew growth (table 2). After 9 years’

exposure, finish No. 21, formulated with

20 percent linseed oil and 10 percent

pentechlorophenol, is rated excellent in

performance followed closely by No. 22,

which contains 5 percent

pentachlorophenol, and No. 23, which

had the linseed oil totally replaced by 25

percent alkyd varnish (the varnish

contained 50 percent solids, so in reality

only 12.5 percent varnish resin is in No.

23). Formulation No. 24 contained 25

Percent of a polyurethane varnish (12.5

pct resin) and did not perform quite es

well as the other three semitransparent

penetrating stains. Generally,

performance of these four finishes

corresponds very well to the

Performance of the FPL natural finish

even after 6 years of exposure since the

first refinish was applied (7).

Differences in performance of the

four modifications of the FPL natural

finish were most pronounced,

particularly on smooth wood surfaces,

after the first 3 years of weathering

(table 5). Smooth wood surfaces are

less absorptive than are rough sawed

5
and show earlier failure of the initial

stain finish. In refinishing, surfaces

initially smooth but roughened by

weathering are more absorptive and

stains performed much the same on all

surfaces. Six years after refinishing, all

the modified FPL stains compare

favorably with the FPL natural finish. In

general, these oil- or varnish-base

semitransparent stains have provided at

least twice the service life of the

transparent WRP finishes. The

pigmented stains, however, obscure

part of the wood grain and do not

provide as natural a finish as do the

WRP’s.

6

Latex Stains

Finish Nos. 7a to 12a were formulated

from acrylic latex resin and pigment to

cover pigment volume concentration

(PVC) of 5 to 30 percent (table 2) (6). In

addition to evaluating the effect of PVC

on stain performance, these stains were

applied over both copper chromatetreated wood (3) and untreated wood.

The copper chromate pretreatment has

been shown to be highly effective in

extending the life of applied finishes

(3,6). All six of these latex stains

performed well over both treated and

untreated wood for 2 years (table 5).

After that, performance on untreated

and particularly untreated smooth wood

declined. After 5 years. however, stain

performance on the treated portion of

all six test panels is still rated very good,

indicating the effectiveness of the

copper chromate surface pretreatment.

When copper chromate was added

directly to a latex stain formulated from

a polyvinyl acetate latex (No. 28) or an

acrylic latex (No. 29), a very durable

finish resulted. After 4 years of

exposure. panels finished with these

two modified stains are rated very good

to excellent. By comparison, a simple

polyvinyl acetate latex-stain system

without added copper chromate has

fallen to a rating of good after 4 years of

exposure.

Water Solutions–

Inorganic Salts

Increased use of waterborne

preservatives for treatment of structural

wood has created an awareness of the

attractiveness of some treatments. In

addition, the efficacy of copper as a

fungicide and chromium in protecting

wood surfaces against degradation by

ultraviolet light, makes waterborne

preservatives containing these metals of

particular interest. The water-soluble

salt finishes currently being evaluated at

the Olympia, Wash., exposure site

include several formulations containing

copper and chromium (copper

chromate).

The copper chromates have been

shown to impart some exceptional

protection to exposed wood in other

studies conducted by FPL (3,6). Finish

formulations based on these earlier

studies were prepared for evaluation in

the study reported here. Finishes

evaluated include three copper

chromate formulations, three

chromated copper arsenate, one

copper pentachlorophenate, one

copper chromate covered with a clear

linseed oil-modified acrylic latex, and

two copper chromates modified with

resorcinol (table 3).

Although copper chromate

treatments impart beneficial mildewcide

and weathering protection properties

when used as exterior wood finishes,

there are some problems due to the

nonuniform appearance of the wood

treated with some solutions. Solutions

containing ammonia are very

nonuniform on redwood, although less

so on western redcedar and Douglas-fir,

particularly during early weathering

months after initial application.

Appearance improves with age. Acid
copper chromate (no added ammonia)

finishes are generally uniform and

attractive initially but are somewhat less

durable than the corresponding

ammoniacal copper chromate finishes.

Performance of all finishes improves

markedly on refinishing.

Of 10 waterborne salt finishes being

evaluated (table 3). those that offer the

best appearance and durability are:

(1) Acid copper chromate (No. 6)

formulated with equal weights of copper

sulfate and sodium dichromate. This

finish is rated very good 6 years after its

second refinishing (table 5). The son

green-brown panels are very uniform

with slight mildew growth. This finish

was first applied in 1966.

(2) Chromated copper arsenate (No.

15) in ammoniacal solution. This finish

has produced an even, son, greenbrown appearance.

(3) Chromated copper arsenates

(Nos. 26 and 27) with a high copper

ratio (compared to No. 15). These

finishes have excellent ratings and have

developed an even green-brown to

brown appearance. Their general

appearance has improved with

exposure and No. 27 has essentially lost

its early nonuniform appearance (7).

The copper chromate and chromated

copper arsenate formulations are

similar to standard wood preservatives

with established performance records.

With these finishes, erosion of the wood

surface is slight compared with that of

other classes of experimental finishes

on exposure.

The ammoniacal copper chromate

with high copper content (No. 5a)

performed very well for the first 5 years

of exposure (table 5). This rating has

fallen to fair with some mildew growth

apparent; panel color is uniform.

however. Performance on Douglas-fir is

still excellent in the sixth year (table 6).

Finish No. 14, also an ammoniacal

copper chromate, has slight mildew

growth but an even green-brown color.

Panels were refinished 6 years ago and

performance is rated as good. Like the

semitransparent stains. the copper

chromate treatments have increased

durability when applied to rough,

weathered wood as compared to

smooth wood, and service lives of at

least 6 years are possible.

Addition of resorcinol to a copper

chromate (Nos. 33 and 34) was

investigated in an attempt to improve

the overall color and evenness of the

finishes. Performance after 4 years of

exposure is rated only fair to good.

Mildew growth is apparent and panels

are an uneven. somewhat unpleasant

brown. A clear. oil-modified acrylic latex

applied over a copper chromate

treatment (without resorcinol) resulted

in an excellent finish rating (No. 30).

Unfortunately, the panel is a dark greenbrown with little of the original wood

color remaining. The treatment was very

effective in protecting against ultraviolet

degradation as was found earlier (3).

The only finish not containing

chromium was the two-component

copper-pentachlorophenate (No. 16).

After 8 years and one refinishing, this

finish shows some blotchiness and

mildew growth. This finish initially

provided a more uniform, natural tan

appearance than did either the copper

chromate or chromated copper

arsenate finishes. Best performance

was on Douglas-fir plywood. Nail

staining and mildew growth detracted

from the performance on redwood.

FINISH PERFORMANCE

ON DIFFERENT SPECIES

The evaluation of natural finishes at

the Olympia, Wash., exposure site was

conducted on three representative

exterior cladding-grade wood siding

materials: Douglas-fir plywood,

redwood board, and western redcedar

board. In general, Douglas-fir plywood

panels used in this study provided the

best substrate for the performance of

the natural finish WRP’s (table 6).

Redwood and western redcedar boards

selected for the study were consistently

more prone to mildew growth.

Performance of the Douglas-fir

plywoods finished with water solutions

of inorganic salts was also better than

for the boards of the other species.

Finish performance with pigmented

stains was essentially consistent for all

three siding materials evaluated.

Finishes on vertically oriented boards

performed better than on horizontal

ones. As indicated earlier, all finishes

performed better when applied to

rough-textured (either rough-sawed or

weathered) surfaces than on smooth

surfaces.

CONCLUSIONS

Three classes of natural wood

finishes with several variations are

undergoing exposure, on three wood

species, to the mild, moist climate of the

Pacific Northwest at Olympia, Wash.

Transparent water-repellent

preservatives (WRP) preserve the

natural color and grain of exposed

wood. Initial treatments last only 2 to 3

years in this climate, but durability and

performance increase to 3 to 5 years

after refinishing. Mildew growth and

unsightly graying are controlled by the

preservative in the formulation. Of those

preservatives examined,

pentachlorophenol was the most

effective. Pentachlorophenol-based

finishes tend to add a golden-brown

cast to the wood surface, while zinc and

cobalt naphthenate-base finishes tend

to slightly bleach the original color of

Douglas-fir. redwood, and western

redcedar.

The second of the natural finish

systems, semitransparent penetrating

stains, offer greater protection to

exposed wood against degradation by

sunlight. These oil-base pigmented

stains promise excellent serviceability.

They change the original color of the

wood to some extent but provide better

durability than do WRP’s.

Increasing the pentachlorophenol

fungicide content from 5 to 10 percent

and decreasing the binder content

(linseed oil or exterior varnish) in oilbase stains from 60 to 20 percent did

not significantly alter performance.

These semitransparent stains have

provided an excellent finish for 6 years

since being refinished. Latex stains,

related to the semitransparent oil-base

stains, do not penetrate wood

substance to any degree but have

performed well, particularly over

surfaces treated with copper chromate

or when copper chromate was added

directly to the formulation.

Water-soluble salts of chromium and

copper (copper chromate) as well as

chromated copper arsenate wood

preservative are excellent natural

finishes and provide outstanding

resistance to mildew attack and to

degradation by sunlight comparable to

the semitransparent stains. They do

change the wood color and initial

finishes may tend to be uneven in

appearance. Appearance improves with

time (especially after refinishing) as the

chemical residue on the wood surface

weathers away. Of the experimental

7
water-soluble salt finishes exposed at

Olympia, a copper chromate and

chromated copper arsenate finish were

found to be the most durable and

attractive. Overall performance ratings

of very good to excellent have been

observed with these water-soluble

finishes 6 years after refinishing.

Except for the latex-based finishes, all

the natural finishes described are

penetrating systems ideally suited for

use on wood siding, particularly if the

siding is rough and/or weathered. The

solvent-based finishes are excellent on

exposed horizontal wood surfaces such

as decks, stairways, and railings. These

penetrating finishes are particularly

attractive and durable on rough-sawed

or weathered wood surfaces, including

LITERATURE CITED

1. American Society for Testing and Materials.

plywood. They do not offer the same

degree of protection from surface

checking as do film-forming impervious

finishes (paints), but they do not blister

or peel even in severe moisture

conditions and are easily refinished. In

all systems evaluated, performance and

durability were increased on refinishing

the weathered wood surfaces.

1978. Annual book of paint-tests for formulated products and applied

coatings. ASTM, 1916 Race Street, Philadelphia, Pa. 19103. ASTM standards.

part 27.

2. Black, J.M., D.F. Laughnan, and E.A. Mraz.

1975. Forest Products Laboratory natural finish. USDA For. Serv. Res. Note

FPL-046, For. Prod. Lab., Madison, Wis.

3. Black, J.M., and E.A. Mraz.

1974. Inorganic surface treatments for weather-resistant natural finishes.

USDA For. Serv. Res. Pap. FPL 232, For. Prod. Lab., Madison, Wis.

4. Feist, W.C.

1979. Weathering of wood in structural use. Proc. of Symp. on Structural Use

of Wood in Adverse Environments, Soc. Wood Sci. and Tech., Vancouver,

B.C. (May, 1978).

5. Feist, W.C., and E.A. Mraz.

1978. Wood finishing: Water repellents and water-repellent preservatives.

USDA For. Serv. Res. Note FPL-0124. For. Prod. Lab., Madison, Wis.

6. Feist, W.C., and E.A. Mraz.

1977. Durability of exterior wood stains. For. Prod. J. 27(1):13-16.

7. Grantham, J.B., T.B. Heebink, J.M. Black, and E.A. Mraz.

1976. Natural exterior finishes for wood in the Pacific Northwest. For. Prod. J.

26(8):21-27.

8. Heebink, T.B.

1970. Performance of exterior natural finishes in the Pacific Northwest. For

Prod. J. 20(3):31-34.

9. U.S. Department of Agriculture.

1975. Wood finishing: Weathering of wood. USDA For. Serv. Res. Note FPL0135, For. Prod. Lab., Madison, Wis.

U.S. GOVERNMENT PRINTING OFFICE: 1 9 8 3 7 5 4 0 4 0 4 0 2 5

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