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Exterior Finishes for Decks

Finish Decks for Optimal Performance

Although Western Red Cedar is a naturally durable wood ideal for decks, its performance is enhanced when protected by an appropriate finish. Decks are subjected to the most challenging exposure conditions. The combination of a horizontal surface, the abrasive effects of foot traffic, pounding rain and full exposure to sun make deck finishing more demanding than other finishing applications such as siding. It is important to ensure that the finish you apply to your Western Red Cedar decking boards has been specifically formulated for this end use. Unseasoned decking should be air dried (not exposed to direct sunlight) and finished on all sides prior to installation.

Choice of Deck Finishes

A full range of both penetrating and film forming finishes are available for cedar decks. Since they do not form a film, the penetrating finishes provide better overall performance and are the easiest to maintain and refinish. Penetrating finishes do not flake, crack, or peel. These treatments help repel water and if formulated with a mildewcide, slow down the growth of micro-organisms like mildew. The penetrating finishes are often used as a natural finish for cedar decks. There are three types of penetrating finishes: water repellents, water repellent preservatives, and solvent-borne semitransparent stains. The best of these finishes are solvent-borne semi-transparent ones.

Transparent Stains and Water Repellents

These are essentially colorless, penetrating finishes that are commonly used for cedar decks. These natural finishes enhance the appearance and service-life of cedar decks. The water-repellent preservatives are preferable to the water-repellents 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 amaging sunlight. To date, the best of them are solvent-borne. 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 water repellent is only one to two years depending on the wood and exposure conditions.

Semi-Transparent Stains

Semi-transparent stains have pigments which provide color and greatly increase the durability of the finish. The pigments help block sunlight and protect the cedar surface, thus increasing service-life. The better products contain water repellents and mildewcides or fungicides. The solvent-borne, semi-transparent stains penetrate into 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 stain formulations currently being developed may correct this problem.

It is possible to switch to a semi-transparent stain when the 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: 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.

This Article was contributed by : Western Red Cedar Lumber Association