Pre-Finished vs. Site Finished

For optimum performance, it is recommended that Western Red Cedar is pre-primed or pre-finished prior to job site delivery.

• Factory-priming As the name implies, this is performed in an industrial setting where a machine coating process applies a primer coating to all six sides of each board. The coated boards are then dried prior to shipment to the job sites. Pre-primed siding and trim boards delivered to a job site should be kept dry and clean prior to installation. All field cuts should be re-sealed with a primer prior to the boards being installed. Top coating over the primer should be completed as quickly as possible as most primers are not intended to be exposed to natural weathering for more than 90 days.
• Factory finishing: This is similar to the above priming process; however, it allows for the additional application of one or two factory applied topcoats of acrylic latex paint or solid stains in job lot quantities and in the colors as selected. Color matching needs to be based on actual samples and a realization that color changes over time due to sunlight and moisture–regardless of which finish is used. It is important not to overdrive nails during installation of factory finished siding, as this will damage the surrounding wood and coating. Overdriven nails must be remedied immediately by filling the depression with exterior grade wood putty or caulk specifically designed for this purpose. Touch up any repairs or field cuts, taking care not to smear the topcoat.

Properly done, factory priming/finishing provides the application of a uniform coating at the recommended coverage of the primer/stain to all six surfaces of each piece (face, back, edges, and ends). It is all done in a controlled environment to permit proper curing of the finish ensuring the wood has a coating to protect it from exposure to sunlight (UV degradation) and surface contamination by mildew and dirt. It can also help with the elimination of lap marks, streaks, and shrink lines after installation. From a construction standpoint, it allows for the ability to install cedar products at any time of the year and often includes a coating warranty.

Photos: Aaron Leitz; courtesy of Western Red Cedar Lumber Association Aaron Leitz

Natural, unfinished cedar is used for a variety of outdoor applications including siding, louvers, screens, and even interiors. Project: Hale Napo’o. Location: Hanalei, Hawaii. Architect: Olson Kundig.

>Finish for Engineered Cedar Products

Engineered wood products have become quite common and popular and that includes cedar products. In virtually all cases, these products are either pre-primed or pre-finished to produce the best results. Such products include clear or knotty finger-joined boards, trim, siding, and paneling. These also include clear Vertical Grain (VG) grade veneer overlay engineered tongue and groove (T&G) products that are used for soffits or other exterior locations. In these cases, the pre-finishing may be available in transparent, semi-transparent, and primer/paint options.

PREPARATION OF CEDAR FOR FINISHING

When site-finishing of cedar is being performed, the first thing to recognize is that the surface condition of the wood on which the finish is applied can substantially affect the performance and life expectancy of the finish. This is true for both newly installed WRC and existing cedar on buildings that needs to be re-finished. Of particular importance for both the first finish and subsequent re-finishing of wood is the need to be clean and dry. Any existing mold and mildew need to be killed with a mild oxygen bleach solution that stays on the wood for thirty minutes to be effective. Note that chlorine bleach destroys lignen in the wood and can be harmful to surrounding plants and gardens.

• New (Not Weathered) Western Red Cedar: New cedar siding and trim should be protected from the weather before, during, and after construction. It is seldom necessary to carry out extensive surface preparation providing the wood has not weathered for more than two weeks and is clean and dry. The moisture content should always be checked as a precaution - the moisture content cannot be above 20 percent. If the wood has been contaminated by dirt, oil or other foreign substances, the contaminants must be removed, and dirt should be washed from the surface. For smooth-planed, flat-grained cedar, the surface should be scuff-sanded with 80 grit sandpaper. This procedure will greatly increase the performance of the coatings but will not detract from a smooth appearance. Surface preparation is not necessary for clean textured cedar.
• Existing (Weathered) Western Red Cedar: Siding and trim that has been exposed to the elements for longer than two weeks may have a degraded surface that is no longer suitable for painting and requires remediation. Preparing the surface by sanding, brushing (with a non-ferrous bristle brush), and washing before applying the finish is recommended. If the wood is gray/dirty, then it needs to be cleaned and sanded to expose fresh fibers. It may be necessary to wash the wood with a restorative cleaner designed to remove graying. These products usually contain oxalic acid. Touch sanding smooth-faced wood with 80-grit sandpaper improves the adhesion of finishes.

Sometimes, a paint or stain supplier will refer to a condition called “mill glaze” on wood which is caused by the heat of the sawmill process interacting with the pitch and resin of different wood species. The suggestion may be made that this needs to be addressed so that the finish will adhere and not be compromised by the mill glaze covering the surface of the wood. However, this is not relevant to cedar because it is free of pitch and resin. Hence, any special preparation to combat mill glaze should not be necessary.

Separate from the above, some manufacturing options do have implications for the best finishing solution.

• Surface Texture: Western Red Cedar products are available with a smooth (i.e., planed) or a textured (resawn, or rough-headed) surface. As a general rule, textured surfaces provide the best mechanical adhesion of the finish to the wood. For smooth-surfaced Western Red Cedar, a two-coat paint system is preferred.
• Moisture Content: Finishing wood that is wet (above 20 percent moisture content) will increase the risk of coating failure. It is best to finish Western Red Cedar when its moisture content has stabilized at the level that will prevail during the service life of the product. In North America, this level is approximately 12 percent moisture content. The best product choice is Western Red Cedar which is kiln-dried at the time of manufacture.

Photos: Hangfei Zhang; courtesy of Western Red Cedar Lumber Association

Clear stain with a UV inhibitor produces dramatic effects while holding up to the harsh Alaskan environment. Project: Aurora Villa.
Location: Fairbanks, Alaska. Architect: Sparano + Mooney Architecture.

Design and Construction Impacts

The performance of wood finishes can be directly impacted by design and construction practices related either to the installation of the cedar or the rest of the building design. While there can be many such factors to be aware of, the following is a representative list:

• Install flashing over all openings and above skirt and band boards.
• Provide adequate overhangs at eaves and gables (designs with no overhangs provide less protection to the finish from UV and moisture exposure).
• Install drip caps over windows and door frames.
• Allow at least 6 inches of clearance between the bottoms of siding and trim boards and the ground line.
• Allow at least 2 inches of clearance between the bottoms of the siding and trim boards and roofing materials.
• Install vapor barriers on the warm side of walls to minimize condensation within walls.
• Vent high humidity areas such as kitchens, laundries and bathrooms.
• Vent attics and crawlspaces.

A general design approach of installing cedar siding over a rain screen greatly decreases the chance that the backs of the siding will get wet and remain wet. This ensures that any water that penetrates the wall assembly will drain down and away from the walls. Rain screens also minimize moisture imbalances within the siding, thus reducing the potential for cupping and extractive bleeding. As a result, coating performance is markedly improved.

FINISHING CEDAR FOR EXTERIOR APPLICATIONS

Exterior applications of finished Western Red Cedar typically include products applied to a vertical surface that is exposed to UV and weather such as siding (bevel, tongue and groove, shiplap, board and batten, etc.) and trim (corner boards, window and door trim, fascia, etc.). It may also include some protected horizontal surfaces such as soffits or porch ceilings which are subject to fewer effects of UV and weather conditions. Conversely it can include some horizontal surfaces that are exposed to more extreme conditions such as decking with heavy foot traffic, the greatest exposure to UV, and to direct rain/snow fall. Similarly, outdoor structures such as canopies, trellises, pavilions, pergolas, etc. are all subject to higher levels of exposure. Therefore, recognizing the degree of protection that is needed for different applications on the exterior is important in order to be realistic about how well or how long a particular finish may last. Of course, that can be balanced with the choice of a desired appearance but educating the building owner about the realistic life expectancy of a finish can go a long way toward setting up reasonable expectations and be a good part of a firm’s risk management practice.

Beyond identifying the degree of exposure, the quality of the finishing product needs to be considered. Western Red Cedar’s excellent finishing characteristics cannot compensate for finishing products that are unsuitable, of inferior quality, or improperlyapplied. Hence when specifying a finish for a Western Red Cedar project it is always best to use a top-of-the-line product from a proven and trusted supplier/manufacturer.