• Select heartwood where possible to minimize nutrient content of wood surfaces and prevent nutrients migrating through the coating to support fungal growth on the surface.

• Round all corners to minimum 5 mm radius to eliminate sharp edges where coating can thin out.

• Prepare surface by sanding with 100 grit sandpaper to physically and chemically activate the surface.  Pretreatment and coating should be applied immediately after sanding. Research shows sanding can double coating life.

• Pretreat with an aqueous formulation containing a UV absorber designed to absorb the visible light that must penetrate transparent coatings to permit the wood to be visible. If the subsequent coating is not completely opaque to UV light, a hindered amine light stabilizer should be added to the visible light protection system. Not only does a visible light protection system prevent degradation of the wood-coating interface, it also prevents release of lignin breakdown products that can be used as a food source by black-stain fungi and prevents light induced breakdown of the biocide components. This pre-treatment must also contain three low-dose carbon-based biocides with differing chemistries to provide cross protection against detoxification and with complementary spectra of activity providing resistance to the full range of black-stain fungi. It should ideally have water repellent properties and must maintain wood surface pH close to neutral or slightly alkaline.

• Apply a transparent water-based catalyzed urethane coating, containing organic and inorganic UV absorbers with absorbance that extends from UVB through to the high-energy part of the visible spectrum (violet light). The coating must virtually eliminate UV from penetrating to the wood, preventing breakdown of wood, biocides and water repellents. This coating will be formulated to be damp-wood friendly to allow application soon after pre-treatment. It will contain no nutrients for fungal growth. It must have an optimum combination of moisture excluding efficiency and vapour permeability to minimize moisture uptake and allow drying after rain. The first coat to be designed to penetrate and bond to the wood, subsequent coats to be designed to ensure maximum intercoat adhesion without sanding between coats. Sufficient coats to be applied to give a film thickness no less than 60 microns to minimize the ability of black-stain fungi to penetrate the film with their infection pegs. The surface layer to have sheeting rather than beading properties to ensure rapid drying after rain or dew, reducing the time available for spore germination.

Additional detailed information on coating wood surfaces has been assembled by the Joint Coatings and Forest Products Committee (http://www.fpl.fs.fed.us/documnts/pdf2004/fpl_2004_bonura001.pdf, 2004).

How long will an exterior wood coating last?  Anywhere from a few months to 20 years or more, depending on the choice of product, how it was applied, and how severe the environment.

Paints tend to last the longest, assuming they are applied properly (see Choosing and applying exterior wood coatings page).  But the range of lifespan for a paint coating is very large.  A low quality product badly applied to a weathered wood surface may barely last two years.  If everything is done right, the coating might last 20 years.  High quality paints and stains generally last longest, and coatings that are in locations protected from sunlight and water tend to last longer.

Stains and water repellents have much shorter lives than paints, but are easier to maintain.  This is one of the reasons they are a popular choice for stairs and decks.  Depending on the degree of exposure to sun, water, foot traffic, and the pigment amount in the stain, expect a life of 1 to 2 years for a stain applied to deck boards and 2 to 5 for a stain applied to products that are not subject to wear.  Water repellents generally last 6 to 12 months.

Results from numerous tests on exterior wood finishes by many experts in this field, particularly by the US Forest Products Lab (USFPL), are summarized below.  See the USFPL link for more information.

Effect of wood anatomy

• Coatings, particularly solid colour stains and paints tend to last longer on dimensionally stable species such as western red cedar, eastern white cedar and Alaska yellow cedar, as these will shrink and swell less than other species and will therefore put less stress on the coating bond.  However deck stains will not last as long on low density species such as western red cedar due to wear.
• Coatings last longer on wood with narrow latewood bands (the dark part of the annual ring) due to density differences between the earlywood (the light part of the ring) and the denser latewood.  The southern pines are characterized by their wide bands of latewood, and therefore these species are considered to be somewhat poor for painting.
• The amount of extractives or resin in wood also affects coating performance. Special primers can be used to block water-soluble extractives, and kiln drying is most effective for fixing resin in wood.  Nutrients in wood can migrate through the coating to support fungal growth on the surface, and heartwood can be chosen to minimize the nutrient content in wood.

Effect of grain

• Finishes last longer on vertical (also called edge grain) versus flat grain, as these surfaces will shrink and swell less and therefore put less stress on the coating bond.  However, it can be difficult to specify type of grain when ordering a product.  Western red cedar and redwood may be available in a premium grade, which will likely be all heartwood, vertical grain.
• If using flat grain, place it bark side out or up if possible, because the grain is less likely to raise on that side, particularly in species with dense latewood bands such as the southern pines, and raised grain is a problem for coating adhesion. This is not an issue when using vertical grain products. Placing bark side out also minimizes checking.

Effect of surface roughness

• Rough-sawn (saw-textured) or roughened wood creates a better coating bond and thicker coating buildup than smooth wood.  The life of a coating can be substantially extended if the wood is roughened.

Effect of sanding

• Sanding (100 grit) can double the life of a coating, for both weathered and freshly planed wood.  This is because sanding removes any damaged surface fibres and also changes the surface chemistry to improve bonding of the coating.

Effect of wood preservatives

• Semitransparent stains last longer when applied to CCA-treated wood – treated wood purchased prior to 2004 was probably treated with CCA.  Research is under way on finishing for wood treated with new preservatives. Protection measures regarding use of treated wood apply when coating preservative-treated wood.

Effect of bluestain

• Bluestain is caused by fungi, and bluestained wood is more permeable than unstained wood, therefore it may absorb more coating.  Make sure to apply sufficient coating.

Effect of weathering

• Sunlight quickly degrades the ability of a wood surface to bond with a coating.  Research has shown a tremendous difference in paint performance on weathered versus unweathered wood.  Paint on boards with no exposure to weather prior to painting lasted at least 20 years.  Boards that had weathered for 16 weeks prior to painting began showing cracks in just 3 years.  For maximum coating life, sand the surface if the wood has been exposed to any sunlight at all, particularly if for more than two weeks.

Effect of product manufacturing

• Plywood:  Coatings on plywood are challenged by the small cracks (face checks) on the surface that are caused by the lathe when the veneer is cut from the log during manufacturing.  As the plywood goes through moisture cycling outdoors, these cracks tend to get larger and stress the coating bond.  Plywood surface, edges and joints in outdoor applications should be protected, and coatings and other products for helping plywood resist cracking can be applied to prevent moisture ingress.  Generally a good stain can effectively protect plywood. Since checking in stained plywood usually occurs during the first six months of outdoor exposure, best coating results can be obtained by applying a first coat and allowing any checking to occur, then six months or so later applying a second coat.  Paints can fail quickly on plywood, unless efforts are made to reduce moisture uptake and also to use flexible products to accommodate dimensional changes of the wood. Roughening the surface is also important. For plywood protection and other issues with plywood, see the recommendations from the Canadian Plywood Association (http://www.canply.org/pdf/main/plywood_handbookcanada.pdf).
• Finger-jointed products: Coatings may perform differently on different parts of these products, as they are not likely to be uniform in grain orientation, in heartwood versus sapwood content, or even in species.  Roughen the surface to extend the life of the coating and minimize these differences. Apply primer and paint all sides if possible to minimize moisture absorption.

Effect of priming

• Field tests have shown that coatings last much longer when a primer coat is used.
• Field tests have shown that siding or shingles last much longer if they are back-primed.

Effect of design and installation

• Use good design and installation practices to protect wood from sunlight and water, and prevent moisture accumulation in wood structures.
• By providing adequate clearance to grade, adequate roof overhang, rainscreen wall and back-priming, the coating life on siding can be effectively extended.
• If using flat grain, place the bark side out if possible to avoid raised grain.
• Use corrosion-resistant fasteners.

Acrylic

A type of water-borne coating product containing acrylic polymers.

Alkyd

A type of polyester resin. Term often used to signify solvent-borne coatings, e.g., oil paints.

Backpriming

The application of a finish coat to the back side of wood such as shingles or siding.

Binder

The non-volatile film-forming solid portion in a coating, which binds the pigment particles together after the film is dry and creates the bond with the substrate.  Typical binders include alkyd resins, acrylic resins and polyurethane resins.

Bleeding

When the colour of a discolouration or other material works up through a coating to the surface.  Commonly used to describe leaching of tannins in extractive species like western red cedar and redwood (typically happens for the first year or so if not stain blocked).

Blistering

When a coating forms bubbles due to air, water vapour or solvent under the film.

Dry lumber

Lumber which has been dried to a moisture content of 19% or less. Any 4” and thinner boards or dimension lumber surfaced at a moisture content (MC) of 19% or less may be stamped “S-DRY” and stamped “KD” if kiln-dried to a maximum moisture content of 19%.  Lumber in the USA may be stamped “KDAT” if kiln-dried after pressure treatment with preservatives.

Enamel

Generic term for an alkyd-based pigmented coating that dries to a smooth, hard, glossy finish.  The term is often more broadly used for a coating which gives a hard, stain-resistant film.

Extractives

Soluble chemicals particularly present in the heartwood of some species which provide the wood with resistance to decay and insects.

Fungicide

A substance which inhibits the growth of fungus.  Often added to coatings to protect the coatings themselves from fungal growth.

Latex

Term used to signify water-borne paints.

Lacquer

Coating material characterized by rapid evaporation of the solvent to produce a thin, hard film.

Linseed oil

Obtained by crushing flax seeds, this natural oil can be used as a vehicle in paints, as a softening agent for the resins in varnishes, or can be used alone as a wood finish material.  Raw linseed oil is a food source for fungi and must be boiled to destroy these nutrients. Most “boiled” linseed oil is not boiled but contains metallic dryers and biocides.

Oil-based paints

Paints using natural oils such as linseed or tung oil as the binder, with turpentine as the usual solvent.  The term is now usually used to refer to paints with both alkyds and oil as the binders, and with a carrier of mineral spirits or other solvents.

Paint

An opaque coating generally made with a binder, liquids, additives and pigments. Applied in liquid form, it dries to form a continuous film that protects and improves the appearance of the substrate.

Pigment

Finely ground solids that impart colour, hiding power (opacity) and ultraviolet protection.

Pitch

Also called resin, this sticky substance is a mixture of rosin and turpentine and is found in most softwoods but particularly the pines, spruces and Douglas-fir.  Can ooze from the pitch pockets and sometimes the knots for a year or two if not set by kiln-drying.  Resin can bleed through finishes and will harden into beads, but this can be cleaned up with mineral spirits and will stop eventually.

Primer

The first complete coat of paint applied in a painting system. Many primers are designed to enhance adhesion between the surface and subsequent topcoats. Most primers contain some pigment, some lend uniformity to the topcoat, some inhibit corrosion of the substrate, and some stop the discolouration of the topcoat.

Resin

For tree resin, see Pitch. In coatings, see Binder.

Sealer

A liquid that seals wood pores so they will not absorb subsequent coats.  Sealers may be transparent, and can act as primers. Some sealers are designed to be left uncoated.

Semi-transparent stain

Stain that alters the natural colour of the wood, yet allows the grain and texture to show through. The term is generally applied to exterior products, but technically applies also to interior wiping stains used for trim, furniture and floors.

Shellac

Alcohol-soluble, clear to orange-coloured resin derived from lac, a substance secreted by insects.  Previously used as a sealer and clear finish for floors, for sealing knots, and in “alcohol-borne” primers; rarely in use anymore. Thinner is denatured alcohol. It is an environmentally friendly product and usually available from finish suppliers.

Solid-colour stain

Exterior stain that obscures the natural colour and grain of wood, but still allows the texture to show through – essentially, a thin paint.

Stain

A coating product which can either be opaque such as a solid colour stain or partly transparent such as a semi-transparent stain. Also refers to wood discolourations such as discolourations caused by tannins in wood extractives, or stain caused by fungi such as bluestain.

Solvent

In generic coatings terminology, refers to the volatile liquid used to improve the working properties of a coating, typically water or hydrocarbons.  In “solvent-borne” coatings, refers specifically to a coating based on hydrocarbons.

Tung oil

Obtained from the nut of the Asian tung tree. Hardly ever used in the raw state as it dries to a non-lustrous finish.  Used in varnishes.

Varnish

Generic term for clear film-forming finish. Transparent or translucent liquids applied as a thin film, which harden.  Can be solvent or water-borne.

VOC

Volatile organic compound.  VOCs are organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere where they may participate in photochemical reactions. They are often associated with solvents, typically considered to be pollutants, and are the subject of regulations in many jurisdictions.

Reference:
Architecture in Wood: A History of Wood Building and Its Techniques in Europe and North America. Hans Jrgen Hansen, Ed., Faber and Faber, London, 1971..