Wood differs from other common building materials in that it is a natural biological material. As such, and because of its chemical and physical makeup, wood is susceptible to deterioration due to the effects of the environment and other agents, the results of which can be diminished by applying suitably formulated protective coatings or treatments. The level of protection required depends upon the extent to which the wood is exposed and the severity of the exposure.
All species of wood in lumber form possess both natural and manufacturing characteristics that affect the choice, application and durability of finishes. Natural characteristics range from seasonal growth patterns, grain and the occurrence of knots, to the exudation of pitch, resin and water soluble extractives. Typical manufacturing characteristics include growth ring orientation resulting from the sawing process, surface texture, type of knot, planing, and moisture content.
Western Red Cedar has unique finish-retention abilities
The most important characteristics of Western Red Cedar (cedar) that contribute to its exceptional ability to accept and retain many different types of finishes are its outstanding dimensional stability, fine texture, a pattern of growth that results in narrow bands of summerwood, and freedom from pitch and resin.
Dimensional stability is related to the wood’s density. The less dense the wood, the less it tends to shrink and swell in response to changes in moisture content. Cedar is a low density wood exhibiting excellent dimensional stability.
Texture refers to the general coarseness of the wood surface, primarily caused by the sawing, planing or sanding process. Pattern of growth, or springwood and summerwood, is the annual increments or bands of tree growth. Cedar has a lower percentage of summerwood than most other species.
Pitch or resin can be found in most softwoods but is absent in cedar. However, water-soluble extractives which give cedar heartwood its natural decay resistance are present.
Western Red Cedar accepts many types of finishes
The choice of an exterior wood finish for cedar depends upon the desired appearance and the degree of protection required. Conversely, the amount of protection provided to the wood depends on the type of finish selected.
Finished wood is a combination of two widely different materials and the properties of both must be considered to achieve the most durable wood-finish system. Note, however, that with all types of finishes, the manufacturer’s recommendations should be followed. Cedar’s excellent finishing characteristics cannot compensate for products that are unsuitable, of inferior quality, or improperly applied. Finishes perform best when the coating is applied to all surfaces (face, back, edges and ends).
In general, finishes for Western Red Cedar can be grouped into four categories :--
- opaque coatings, such as paints and solid colour stains
- semi-transparent stains
- natural finishes such as water-repellents and water-repellent preservatives
In some respects, wood preservatives and fire-retardant coatings may also be classified as finishes but are not part of our finishing and coating processes.
Paint provides the most surface protection against weathering and wetting by water while providing colour and concealing some of the wood’s characteristics. Although paint can reduce wood’s absorption of water, paint itself is not a preservative. Alkyd oil-based primers usually offer the best shield against discoloration by water-soluble extractives. Latex paint, particularly 100% acrylic formulations, remain more flexible with age and are better able to accommodate dimensional changes by stretching and shrinking with the wood.
Solid color stains are opaque finishes with fewer solids than paint.Available in a wide spectrum of hues, solid colour stains obscure the wood’s true colour but allow some of the natural characteristics and texture of cedar to remain. Solid colour stains perform best on textured surfaces. They are non-penetrating and, like paints, form a film. A stain-blocking primer should be applied first, followed by a 100% acrylic latex-based top coat.
Water-repellents and water-repellent preservatives may be applied to cedar used above ground. These formulations reduce water absorption in the short term. The addition of a fungicide that inhibits the growth of mildew and decay fungi will further increase wood’s durability.
A low-wax-content water-repellent preservative applied to newly-milled cedar as a single-coat pre-treatment before painting may help reduce discoloration caused by bleeding of water-soluble extractives.
Semi-transparent stains may be latex or oil-based. The semi-transparent nature of the stain, due to its low solids content, does not block all ultraviolet radiation and some will reach the wood’s surface. Latex stains do not penetrate the surface and are not as durable.
Caution: Transparent, non-flexible, film-forming finishes such as lacquer, shellac, urethane, and varnish are not recommended for exterior use on cedar. Ultraviolet radiation can penetrate the transparent film and degrade the wood. Regardless of the number of coats, the finish will eventually become brittle, develop severe cracks and then fail.