In terms of application, here are some of the things to be sure are specified or called for in all exterior situations:

• Keep cedar protected from the elements on the job site.
• Acclimatize Western Red Cedar to local humidity conditions before installing.
• Proper seasoning of the wood must be done to accept the finish if it is not already seasoned to an acceptable moisture content.
• Finishes should be applied as soon as the wood is dry.
• Use stainless steel, double hot-dipped galvanized or aluminum fasteners to avoid staining.
• Apply primer or stain to all six sides, including surfaces exposed after they are field cut, before installing. Note that the end grain may require 2 coats to fully protect it.

Based on all of the above, the following options can be considered for exterior finishing of cedar recognizing that each one has its own service life, benefits, and challenges.

Photos: Julian Parkinson; courtesy of Western Red Cedar Lumber Association

For finishing, the design team opted to let the home weather naturally, eventually turning an elegant silvery grey. It’s a design decision that highlights cedar’s versatility as well as its durability in the face of a salty coastal environment. Project: Five Cove House. Location: Marvin’s Island, Nova Scotia, Canada. Architect: RHAD Architects.

Unfinished Cedar

Western Red Cedar performs satisfactorily as a decking and siding product if it is left unfinished to weather naturally. Some design professionals or owners choose this option based on any of the following:

• A preference for the silvery grey look of naturally weathered Western Red Cedar – unfinished WRC creates a natural gray colored patina on the wood over time.
• A desire not to invest the time and effort required to maintain the finish on a cedar product.
• Project inaccessibility, causing difficulty in maintaining the finish on a Western Red Cedar project, for example, siding on very tall buildings.
• A concern about the environmental impact of applying a finish on Western Red Cedar.

These are all valid considerations, and the reality is that Western Red Cedar siding and decking can have a long service life without a finish coating in many environments — as long as good design, installation and maintenance practices are followed. Without the protection provided by a coating against moisture intrusion, (especially end grain sealing), steps must be taken during the installation process to allow Western Red Cedar to readily dry following exposure to moisture, otherwise the risk of decay increases.

It is also important to understand that the choice not to apply a finish to an exterior cedar product has long-term implications. As Western Red Cedar weathers, it will lose its natural color and become grey. In many climates, it weathers to a silvery grey color. In other climates, because of varying moisture and sun exposure conditions, Western Red Cedar may not weather uniformly. In some cases, it may develop a brownish tint. In order to restore weathered cedar to a state where it can be painted or stained, significantly more effort will be required to prepare the surface of the wood for coating. Note that unfinished cedar left to weather for just 12 weeks loses 50 percent of its ability to hold a film forming finish. That is why it is highly recommended that any finishing is done within 2 weeks of installation. Factory finishing is encouraged and recommended.

If the choice is made not to apply a finish to Western Red Cedar, contaminants such as dirt and mildew should be regularly removed to maintain its beautiful, natural appearance. Life Cycle Assessment (LCA)1 of Western Red Cedar siding and decking shows it has the most benign environmental impact of all the alternative product choices for those building applications. Coatings, especially the solvent borne finishes and the frequency of their application negatively impact the LCA of cedar siding and decking. This impact may influence a decision not to apply a finish. Fortunately, a new generation of more environmentally friendly, water borne, coatings are emerging, and these products should be considered.

Clear/Transparent Finishes

These are essentially colorless, penetrating finishes that are commonly used for cedar decks to enhance the appearance and service-life. The water-repellent preservatives are preferable to the water-repellents only because they contain a mildewcide and often other fungicides that help inhibit surface mildew growth. Many of these water-repellants contain ultraviolet light blockers or absorbers that help protect the Western Red Cedar from damaging sunlight. To date, the best of them are solvent-borne (oil based) but new products containing water soluble oils are becoming more readily available. The water repellent preservative finishes are very effective in stopping the absorption of liquid water thus reducing swelling and shrinking of the wood. Water repellents should be used only on newly manufactured bare cedar, on restored bare cedar, or on cedar previously treated with the same type of product.

When used as a natural finish, the service life of a clear water repellents is about one year, depending on the weather exposure. Therefore, frequent reapplication is necessary to maintain the original look, which may prove difficult if the finish wears unevenly allowing the color of the wood to change. Accordingly, architects should discourage the use of transparent finishes on siding and trim to prevent unrealistic expectations with regards to maintenance. The exception is for protected areas such as ceilings and walls of covered porches, soffits, etc. These areas can use transparent finish if they aren’t receiving any sun or weather exposure so the service life can be much longer. Reportedly, 6-8 or even 10 years is possible in these protected applications.

Note also that transparent, non-flexible, film-forming finishes such as lacquer, shellac, urethane, and varnish are not recommended for exterior use on Western Red Cedar. Regardless of the number of coats, the finish will eventually become brittle, develop cracks in the coating film and then fail.

Image courtesy of realcedar.com

A comparison of the expected service life of different types of finishes on Western Red Cedar applied to building exteriors. Source: www.realcedar.com/static/2d35a974acbf517df55a40d13ed6d7a4/How-to-Finish.pdf.

Semi-Transparent Finishes

Semi-transparent stains have pigments that provide color and greatly increase the durability of the finish. The pigments help block sunlight and protect the cedar surface. The better products contain water repellents and mildewcides or fungicides. The solvent-borne, semi-transparent stains penetrate the cedar without forming a film and do not blister or peel even if excessive moisture enters the cedar. However, latex semi-transparent stains can sometimes build up a film, especially on recoating, which can lead to peeling. Latex formulas containing water-soluble oils are becoming more readily available.

It is possible to switch to a semi-transparent stain when a deck needs to be refinished. Even if the deck has been coated with a clear or transparent stain for many years, an oil-based semi-transparent stain can be applied, provided the wood surface is cleaned. Note, however, that film-forming finishes are normally not recommended for use on Western Red Cedar decks because they can fail by cracking, flaking, and peeling and can be very difficult to refinish and maintain.

Semi-transparent stains can be applied by brush, spray, pad, or roller. Brushing will usually give the best penetration and performance. Spray or roller application followed by back-brushing is also an acceptable method. Oil-based stains are generally thin and runny, so the application can be messy. Lap marks can be prevented by staining continuous lengths. This method prevents the front edge of the stained area from drying before a logical stopping place is reached. Working in the shade is desirable because the drying rate is slower. Stain that has been applied by spray, without back-brushing, is prone to show blotchy patterns as it weathers. Two coats of penetrating oil-based stain on textured Western Red Cedar will provide a longer service life than one coat, but only if the wood will accept the second coat. Stir the stain thoroughly during application to prevent settling and color change. Avoid mixing different brands or batches of stain.

Latex semi-transparent stains do not penetrate the wood surface, but they are easy to apply and less likely to form lap marks. These stains are film-forming and may not be as durable as oil-based stains. They can be applied in one coat on smooth face cedar or two coats on textured face cedar with a longer service life of approximately two to four years. Keep in mind that the building design and the exposure to weather will impact the service life of semi-transparent finishes too.

Photos: Bruce Damonte; courtesy of Western Red Cedar Lumber Association

A semi-transparent grey stain was very successfully used on the exterior of this school while a semi-transparent whitewash stain was used on the interior ceiling. Project: French International–Middle School. Location: Portland, Oregon. Architect: Hacker.