Selection and Application of Exterior Finishes for WoodDaniel L. Cassens, Department of Forestry and Natural Resources, Purdue University, and

Willam C. Feist, Forest Products Laboratory, Forest Service, U.S. Department of Agriculture

All too often, exterior finishes for wood are short-Iived in durability or fail completely. Failure isusually the result of the wrong kind of finish beingapplied to the wood surface or of not followingrecommended application procedures. In eithercase, the homeowner is faced with the expensiveand time consuming problem of refinishing. instruc-tion should be given in selecting the appropriatefinish/wood combination and in following throughby applying the finish as specified by the manufac-turer. This publication will discuss how to select theappropriate finish for the many different types ofwood products currently used on structures as wellas correct application procedures.

TYPES OF WOOD PRODUCTSThere are three general categories of wood

products commonly used in construction. These are(1) lumber, (2) plywood, and (3) reconstituted woodproducts, such as hard board and particleboard.Each product has unique characteristics which willaffect the durability of any finish applied to it.

LumberLumber is being used less and less as exterior

siding, but was once the most common woodmaterial used in construction. Many older homeshave wood siding. The ability of Iumber to retain andhold a finish is affected by species, by graindirection or how the piece was sawn, and bysmoothness.

The weight of wood varies tremendously betweenspecies. Some common construction woods such assouthern yellow pine are dense and heavy withrespect to the lighter weight ones such as redwoodand cedar. The weight of wood is important because

heavy woods shrink and swell more than light ones.This dimensional change in lumber occurs as thewood gains or loses moisture. Wood in heatedhomes tends to dry and shrink in the winter andgains moisture and swells in the warm summermonths. Excessive dimensional change in woodconsistently stresses a paint film and may result inearly failure.

Some species have wide bands of dark, densewood alternating with bands of light, softer wood.The light bands are formed in the spring of the yearand are called springwood or earlywood. The darkbands are formed in the summer and are calledsummerwood or latewood (Figure 1). Bands ofsummerwood are characteristic of southern yellowpine and some boards of Douglas-fir. Paint will nothold well on these species, and penetrating stainsare preferred. On the other hand, wide summerwoodbands are not characteristic of redwood and cedar;therefore, these species are preferred when paintwill be used.

Grain direction also affects paint-holding char-acteristics and is determined at the time lumber iscut from a log. Lumber is referred to as flat-grained,edge-grained (quartersawed) or a combination ofthe two (Figure 2). Most standard lumber contains ahigh percentage of flat grain. Lumber used for boardand batten siding, drop-siding or shiplap is fre-quently flat-grained. Bevel siding of redwood orcedar is often, but not always, manufactured so thatit is edge-grained. Edge-grained bevel siding willhold paint well. Flat-grained lumber will not holdpaint as well since it shrinks and swells more thanedge-grained lumber and because wide, dark bandsof summerwood are frequently present (Figure 3).

Paint will last longer on smooth, edge-grained

Figure 1, Spring and summerwood bands in southernyellow pine. These distinct bands often lead to early

paint failure. Therefore, penetrating stares are preferred

Figure 2. Edge-grained (or quartersawed)board (left) and flat-grained (or plainsawed)board (right) cut from a log

Figure 3. Paint applied over edge-grainedboards (top and bottom) performs better than that

applied to flat-grained boards (middle).

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surfaces. Penetrating stains or preservative treat-ments are preferred for rough sawn lumber. Thesetreatments often accentuate the natural or rusticlook of rough sawn lumber and allow the wood grainand surface texture to show through the finish.

PlywoodExterior plywood with a rough sawn surface is

commonly used for siding. Smooth sanded plywoodis not recommended for siding, but it may be used insoffits. Sanded and rough sawn plywood willdevelop surface checks, especially when exposed tomoisture and sunlight. These surface checks canlead to early paint failure with oil or alkyd paintsystems (Figure 4). Quality acrylic latex paintsystems generally perform better. The flat-grainedpattern present in nearly all plywood can alsocontribute to early paint failure. Therefore, if smoothor rough sawn plywood is to be painted, specialprecautions should be exercised. Penetrating stainsare often more appropriate for both smooth sandedand especially rough sawn exterior plywoodsurfaces.

Figure 4. Early paint failure on plywood due topenetration of moisture into surface checks of plywood

Plywood manufactured with a medium densitypaper overlay is frequently called MDO. Comparedto standard rough sawn or smooth plywood, MDOholds paint well, MDO plywood is not always a stockitem in many lumber yards, but it can usually beordered.

Reconstituted Wood ProductsReconstituted wood products are those made by

forming small pieces of wood into large sheetsusually 4 by 8 feet or as required for a specialized usesuch as beveled drop siding. These products may beclassified as fiberboard or particleboard, dependingupon the nature of the basic wood component.

Fiberboards are produced from mechanical pulps.Hardboard is a relatively heavy type of fiberboardand is used for exterior siding. It is often sold in 4 by8 foot sheets and as a substitute for beveled dropsiding, traditionally made from solid wood.

Particleboard are manufactured from wholewood in the form of splinters, chips, flakes orshavings. Waferboard and flakeboard are two typesof particleboard made from relatively large flakes orshavings.

Only some fiberboards and particleboard aremanufactured for exterior use. Film-forming fin-ishes such as paints and solid color stains will givethe most protection to these reconstituted woodproducts. Some reconstituted wood products maybe factory primed with paint, and some may evenhave a top coat.

TYPES OF WOOD FINISHESPaint

Paints are common coatings used on wood andprovide the most protection. They come in a widerange of colors and may be either oil or latex based.Latex-based paints and stains are water borne, andoil or alkyd paints are solvent borne. Paints are usedfor aesthetic purposes, to protect the wood surfacefrom weathering and to conceal certain defects.

Paints are applied to the wood surface and do notpenetrate it deeply. The wood grain is completelyobscured, and a surface film is formed. This surfacefilm can blister or peel if the wood is wetted or ifinside water vapor moves through the house walland wood siding because of the absence of a vaporbarrier.

Latex paints are generally easier to use sincewater is used in clean-up. They are also porous and,thus, will allow some moisture movement. Incomparison, oil-based paints require organicsolvents for clean-up, and some are resistant tomoisture movement,

Paints perform best on smooth, edge-g rainedlumber of lightweight species such as redwood andcedar. Paints are the only way to achieve a brightwhite finish. However, they are subject to peelingwhere moisture is a problem.

Solid Color StainsSolid color stains, also called heavy bodied stains,

are opaque finishes which come in a wide range ofcolors and are defined as stains. Solid color stainsare made with a much higher concentration ofpigment than the semitransparent penetratingstains. As a result they will obscure the natural woodcolor and grain. Oil-based solid color stains tend to

form a film much like paint and as a result can alsopeel loose from the substrate. Latex-based solidcolor stains are also available and form a film as dothe oil-based solid color stains. These stains aresimilar to thinned paints.

Semitransparent Penetrating StainsSemitransparent penetrating stains are growing in

popularity. They are moderately pigmented and,thus, do not totally hide the wood grain. These stainspenetrate the wood surface, are porous, and do notform a surface film like paints. As a result, they willnot blister or peel even if moisture gets into thewood. Penetrating stains are alkyd or oil-based, andsome may contain a fungicide or water repellent,Latex-based (water borne) stains are also available,but they do not penetrate the wood surface as do theoil-based stains.

Stains are most effective on rough lumber orplywood surfaces, but they also provide satisfactoryperformance on smooth surfaces. They are availablein a variety of colors and are especially popular inthe brown tones since they give a “natural or rusticwood appearance. ” They are not available in white.They are also an excellent finish for weatheredwood. They are not effective when applied over asolid color stain or over old paint.

Water-Repellent PreservativesA water-repellent preservative may be used as a

natural finish for wood surfaces. It contains a woodpreservative, a small amount of wax as a water repel-lent, a resin or drying oil, and a solvent such as tur-pentine or mineral spirits. Water-repellent preserva-tives do not contain any coloring pigments. Therefore,the resulting finish will vary in color depending uponthe kind of wood. The preservative prevents woodfrom graying by inhibiting mildew.

Water-repellent preservatives may also be used asa treatment for bare wood before priming andpainting or in areas where old paint has peeled,exposing bare wood, particularly around butt jointsor in corners, This treatment keeps rain or dew frompenetrating into the wood, especially at joints andend grain, and thus decreases the shrinking andswelling of wood. As a result, less stress is placed onthe paint film, and its service life is extended (Figure5). This stability is achieved by the small amount ofwax present in water-repellent preservatives, Thewax also decreases the capillary movement of waterup the back side of lap or drop siding. The fungicideinhibits surface decay.

Be sure to purchase the correct type of water-repellent preservative. Any type of water-repellentpreservative can be used as a natural exterior finishby itself, but only some are paintable. Manufacturers

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have also developed water-repellent preservativesspecifically for exterior finishes.

Water repellents are also available. These aresimply water-repellent preservatives with thepreservative left out. Water repellents are not goodnatural finishes but can be used as a stabilizingtreatment before priming and painting.

Before purchasing and using a water-repellentpreservative or water repellent, read the labelcarefully and follow the manufacturer’s directions.

VarnishesVarnishes, synthetic resins and other clear film-

forming finishes provide an attractive finish forwood since they allow the natural wood color andgrain to show through. Unfortunately, the durabilityof these finishes on wood under the action ofsunlight and moisture is limited. Regardless of thenumber of coats applied, the film will begin to crackand peel, and the finish will have to be completelyremoved by sanding or with a varnish removerbefore a new coat is added. In severe exposures,clear film finishes may last barely one year. Alternatefinishes such as semitransparent stains and water-repellent preservatives will give a longer service lifeand are easier to refinish.

Exterior marine or spar varnishes may be usedwith some success on exterior doors and other areasif adequate protection from the weather is provided.

PreservativesWood preservatives are not considered to be fin-

ishes. However, wood properly treated with a preser-vative can withstand years of exposure to severedecay and insect attack without being affected. Thecommon wood preservatives are creosote, penta-chlorophenol in oil, and the newer water-borne salttreatments—all of which are restricted-use pesti-cides (see Warning on back page). Creosote andpentachlorophenol in oil result in a dark and oily sur-face. Odor with creosote is a problem. Wood treatedwith creosote or pentachlorophenol in oil is not recom-mended for use around the home where people willcome in contact with it. However, wood treated withwater-borne salts is suggested for use as patiodecks, outside steps, privacy fences and other homeuses. This material is generally light to bright green orbrown in color. It can be used outdoors without finish-ing and will go practically unchanged or weather to alight gray

APPLICATION OF WOOD FINISHESPaint

Proper surface care and preparation beforeapplying paint to wood is essential for good perfor-mance. Wood and wood-based products should beprotected from the weather and wetting on the jobsite and after they are installed. Surface contamina-tion from dirt, oil and other foreign substances mustbe eliminated. It is best to paint wood, surfaces within2 weeks, weather permitting, after installation.

To achieve maximum paint life, follow these steps1. Wood siding and trim should be treated with a

paintable water-repellent preservative or waterrepellent. Water repellents protect the woodagainst the entrance of rain and dew and thusprevent swelling and shrinking. They can beapplied by brushing or dipping. Lap and butt

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joints and the ends of panel products such asplywood, hardboard and particleboard should beespecially well treated since paint normally failsin these areas first (Figure 6). Allow at least twowarm, sunny days for adequate drying beforepainting the treated surface. If the wood has beendip-treated, allow at least one week of favorableweather.After the water-repellent preservative or waterrepellent has dried, the bare wood must beprimed. Since the primer coat forms a base for allsucceeding paint coats, it is very important. Forwoods with water-soluble extractivess such asredwood and cedar, the best primers are goodquality oil-based and alykd-based paints. Somelatex-based primer paints are also designed foruse over these woods. The primer seals in theextractivess so that they will not bleed through thetop coat. A primer should be used whether the topcoat is an oil-base or latex-base paint. For specieswhich are predominately sapwood and free ofextractives, such as pine, a high quality acryliclatex paint may be used as both a primer and topcoat. Enough primer should be applied toobscure the wood grain. Do not spread the primertoo thinly. Follow the spreading rates recom-mended by the manufacturer. A primer coatwhich is uniform and of the proper thickness willdistribute the swelling stresses which develop inwood and thus prevent premature paint failure.Two coats of a good-quality acrylic latex housepaint should be applied over the primer. If it is notpractical to apply two top coats to the entirehouse, consider two top coats for fully exposedareas on the south and wast sides. Areas fullyexposed to sunshine and rain are the first todeteriorate and therefore should receive two

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coats. On those wood surfaces best suited forpainting, one coat of a good house paint over aproperly applied primer (a conventional two-coatpaint system) should last 4 to 5 years, but twocoats can last up to 10 years.One gallon of paint will cover about 400 squarefeet of surface area. However, coverage can varywith different paints and application proceduresResearch has indicated that the optimum thick-ness for the total paint coat (primer and two topcoats) is 4-6 mils or about the thickness of a sheetof newspaper. The quality of paint is usually, butnot always, related to the price. Brush applicationis always superior to roller application.

Additional tips on paintingTo avoid future separation between paint coats,

the first top coat should be applied within 2 weeksafter the primer and the second coat within 2 weeksof the first. As certain paints weather they can form asoap-like substance on their surface which mayprevent proper adhesion of new paint coats. If morethan 2 weeks elapse before applying another paintcoat, scrub the old surface with water using a bristlebrush or sponge. If necessary, to remove all dirt anddeteriorated paint, use a mild detergent. Then rinsewell with water, and allow the surface to dry beforepainting,

To avoid temperature blistering, oil-base paintsshould not be applied on a cool surface that will beheated by the sun within a few hours. Temperatureblistering is most common with thick paint coats ofdark colors applied in cool weather. The blistersusually show up in the Iast coat of paint and occurwithin a few hours to 1 or 2 days after painting Theydo not contain water.

Oil-base paint may be applied when the temper-ature is 40° F or above. A minimum of 50°F is desiredfor applying latex-based paints. For proper curing ofthe paint film, the temperature should not dropbelow 50°F for at least 2 hours after paint applica-tion. Low temperatures will result in paint failure.

To avoid wrinkling, fading or loss of gloss of oil-base paints and streaking of latex paints, the paintshould not be applied in the evenings of cool springand fall days when heavy dews form before thesurface of the paint has thoroughly dried.

Solid Color StainsSolid color stains may be applied to a smooth

surface by brush or roller application, but brushapplication is best. These stains act much like paint.One coat of solid color stain is adequate, but twocoats will provide better protection and longerservice.

Unlike paint, Iap marks may form with a solid colorstain. Latex-based stains are fast-drying and aremore likely to show lap marks than the oil-based

ones. To prevent lap marks follow the proceduressuggested under application of semitransparentpenetrating stains.

Semitransparent Penetrating StainsSemitransparent penetrating stains may be

brushed or rolled on. Brushing will give betterpenetration and performance. These stains aregenerally thin and runny, so application can bemessy. Lap marks will form if stains are improperlyapplied (Figure 7). Lap marks can be prevented bystaining only a small number of boards or a panel atone time. This method prevents the front edge of thestained area from drying out before a logicalstopping place is reached. Working in the shade isdesirable because the drying rate is slower. Onegallon will usually cover about 300-400 square feetof smooth surface and from 150-200 square feet ofrough surface.

For long life with penetrating oil-base stain onrough sawn or weathered lumber, use two coats andapply the second coat before the first is dry. Applythe first coat to a panel or area as you would toprevent lap marks. Then work on another area sothat the first coat can soak into the wood for 20 to 60minutes. Apply the second coat before the first coathas dried. (If the first coat dries completely, it willseal the wood surface so that the second coat can-not penetrate into the wood.) About an hour afterapplying the second coat, use a cloth or sponge towipe off the excess stain that has not penetrated intothe wood. Stain which did not penetrate will form anunsightly surface film and glossy spots. Avoidintermixing different brands or batches of stain. Stirstain thoroughly during application. Note: Spongesor cloths that are wet with oil-base stain areparticularly susceptible to spontaneous combus-tion. To prevent fires, bury them, immerse them inwater, or seal them in an airtight container imme-diately after use.

A two-coat wet system on rough wood may last aslong as 10 years in certain exposures. If only onecoat of penetrating stain is used on new wood, itsexpected life is 2 to 4 years, but succeeding coatswill last longer.

Water-Repellent PreservativesThe most effective method of applying a water-

repellent preservative is to dip the entire board intothe solution. However, brush treatment is alsoeffective. When wood is treated in place, liberalamounts of the solution should be applied to all lapand butt joints, edges and ends of boards andpanels. It is important to apply liberal amounts of thesolution to the end grain of wood. Areas especiallyvulnerable to moisture, such as the bottoms of doorsand window frames, should not be overlooked. One

gallon will cover about 250 square feet of smoothsurface or 150 square feet of rough surface. The lifeexpectancy is only 1-2 years, depending upon thewood and exposure. Treatments on rough surfacesare generally longer-lived than those on smoothsurfaces. Repeated brush treatment to the point ofrefusal will enhance durability and performance.

REFINISHINGPaint

If you are refinishing an old paint coat, propersurface preparation is essential if the new coat is togive the expected performance. First, scrape awayall loose paint. Use sandpaper on any remainingpaint to “feather” the edges smooth with the barewood. Then scrub any remaining old paint with abrush or sponge and water. Rinse the scrubbedsurface with clean water. Wipe the surface with yourhand. If the surface is still dirty or chalky, scrub itagain using a detergent. Mildew should be removedwith a dilute household bleach solution. Rinse thecleaned surface thoroughly with fresh water andallow it to dry before repainting. Areas of exposedwood should be treated with a water-repellentpreservative, or water repellent, and allowed to dryfor at least two days, and then primed. Topcoats canthen be applied.

It is particularly important to clean areas pro-tected from sun and rain such as porches and sidewalls protected by overhangs. These areas tend tocollect water soluble materials that interfere withadhesion of the new paint. It is probably adequate torepaint these protected areas every other time thehouse is painted.

Latex paint can be applied over freshly primedsurfaces and on some surfaces where an oil-basepaint has already been used and weathered. Whereold surfaces are to be repainted with latex paint asimple test should be conducted first. After cleaningthe surface, repaint a small, inconspicuous area withlatex paint, and allow it to dry at least overnight,Then, to test for adhesion, firmly press one end of a“band aid” type adhesive bandage onto the paintedsurface. Jerk it off with a snapping action. If the tapeis free of paint, it tells you that the latex paint is wellbonded and that the old surface does not needpriming or additional cleaning (Figure 8). If the newlatex paint adheres to the tape, the old surface is toochalky and needs more cleaning or the use of an oil-base primer.

Solid Color StainsThe same technique used to restore a paint coat

can be used for solid color stains.

Semitransparent Penetrating StainsSemitransparent penetrating stains are relatively

easy to refinish. Excessive scraping and sanding arenot required. Simply use a stiff bristle brush toremove all surface dirt, dust, and loose wood fibers,and then apply a new coat of stain. The second coatof penetrating stain often lasts longer since itpenetrates into small surface checks which open upas wood weathers.

Water-Repellent PreservativesWater-repellent preservatives can be renewed by

a simple cleaning of the old surface with a bristlebrush and an application of a new coat of finish. Todetermine if a water-repellent preservative has lostits effectiveness, splash a small quantity of wateragainst the wood surface. If the water beads up andruns off the surface, the treatment is still effective. Ifthe water soaks in, the wood needs to be refinished.Refinishing is also required when the wood surfaceshows signs of graying.

Note: Steel wool and wire brushes should not beused to clean surfaces to be finished with semitrans-parent stains or water-repellent preservatives sincesmall iron deposits may be left behind. Pentachloro-phenol may cause iron remaining on the surface tocorrode. The corrosion products may then react withcertain wood extractivess to form a dark-blue, unsight-ly discoloration which becomes sealed beneath thenew finishing system, Pentachlorophenol was com-monly used in some semitransparent penetratingstains and water-repellent preservatives before itbecame a restricted-use pesticide.

Acknowledgement is given to the Forest Products Laboratory, Forest Service, U.S. Department of Agriculture, Madison, Wisconsin forfinancial assistance in support of the preparation of this publication.

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WARNINGDo not mix bleach with ammonia or with any detergents or cleansers containing ammonia!

Mixed together the two are a lethal combination, similar to mustard gas. In several instancespeople have died from breathing the fumes from such a mixture. Many household cleanerscontain ammonia, so be extremely careful with what types of cleaners you mix bleach.

Use caution with wood finishes which contain pesticides. When used improperly they can beinjurious to man, animals and plants. For safe and effective usage, follow the directions, andheed all precautions on the labels. It inadvisable to wear unlined protective gloves and to covernearby plant life when using any material containing pesticides.

Avoid spraying a pesticide wherever possible. Drift from a pesticide, applied as a spray, maycontaminate the surrounding environment.

Store finishes containing pesticides in original containers under lock and key—out of reachof children and pets — and away from foodstuffs. Follow recommended practices for thedisposal of surplus finishing materials and containers.

Note: Registrations of pesticides are under constant review by the Environmental ProtectionAgency and the Department of Agriculture. Use only pesticides that bear a Federal registrationnumber and carry directions for home and garden use. Since the registration of pesticides isunder constant review by State and Federal authorities, you should consult with a responsibleState agency as to the current status of the pesticides discussed in this report.

Wood preservative warning. Because the preservatives creosote, pentachlorophenol, andwater-borne salt treatments are restricted-use pesticides, only the wood treated with these mate-rials may be sold to the general public. Suppliers of such treated lumber can provide EPA-approvedconsumer information sheets on its proper use and handling. Other preservatives classified asgeneral-use pesticides are available to the public.

Sponsored by the Cooperative Extension Services ofIllinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota,

Ohio, South Dakota, Wisconsin, and Lincoln University-Missouri.Printed and distributed in cooperation with Extension Service, U.S. Department of Agriculture, Washington, D.C.

Programs and activities of the Cooperative Extension Service are available toall potential clientele without regard to race, color, sex, age, national origin, or handicap.

In cooperation with the North Central Region Educational Materials Project.

Issued in furtherance of Cooperative Extension work, Acts of Congress of May 8 and June 30, 1914,in cooperation with the U.S. Department of Agriculture and Cooperative Extension Services of Illinois,

Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin.H.A. Wadsworth, Director, Purdue University Cooperative Extension Service, West Lafayette, IN 47907.

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