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.