Preserving Outdoor Wood
Staining and sealing outdoor wood is among the best ways to protect it from the elements.
By Matt Weber
Whether it’s pressure-treated, cedar, cypress, redwood or even a high-end exotic hardwood, the right care and maintenance will protect exterior wood and keep it in good shape for years. Staining and sealing outdoor wood is among the best ways to protect it from the elements.
Know Your Coatings
The most common outdoor wood coating is an exterior-grade penetrating stain. These water-repellent preservatives include a mildewcide, and some products contain ultraviolet light absorbers that protect from sun exposure. Available in both oil- and water-based formulations, the resins penetrate wood pores to provide pigment and block out the damaging effect of weather while allowing the natural woodgrain and texture to shine through.
The second category is a film-forming sealant that bonds to the surface of the wood like paint or shellac. These products provide a high-gloss furniture look, while still allowing the natural grain to show through. Available in oil- or water-based finishes, they form a durable beautiful, satin surface, but they can only be maintained with another coat of film-forming sealant. Pigments are added to change the wood color and add UV protection. Film-forming sealants should be avoided in areas exposed to foot traffic, because the abrasion can wear through the film coating.
Outdoor wood coatings are typically formulated with either water or oil. Most water-based stain/sealants have tiny particles of pigment and resin that adhere to each other very tightly as the finish dries, similar to a patchwork quilt. With oil-based finishes, the tiny particles actually fuse together chemically into one large sheet-like substance, which achieves a harder finish and is less likely to develop an amber color tone. Examine the product’s label for clues to the coating’s quality, looking for any reference to “non-yellowing” properties.
Water-based finishes are generally heralded for their ease of use. Compared to oil-based formulas, they’re easier to clean up, have a lower odor and are often less expensive. However, most water-based coatings require more coats and still don’t last as long as their oil-based or “alkyd” counterparts, which generally provide more long-term, wood-preserving durability.
Allow new pressure-treated wood to dry before staining or sealing. The treatment of lumber with waterborne preservatives leaves moisture in the wood.
This is why fresh PT lumber often arrives wet from the supplier, and the moisture can impede the penetration of stains and paints. For best performance of paint and stain coatings, allow the treated wood to dry for 2 to 4 weeks prior to application. Estimating exactly how long treated wood will take to dry is difficult, and a lot depends on how much time has elapsed since the treatment, the lumber’s exposure to the sun, ambient weather, etc.
Wood with natural preservatives, such as Western Red Cedar, cypress and redwood, do not require as much drying time because the wood was never pressure-treated with a preservative.
Contrary to popular belief, new wood still needs to be cleaned to remove any “mill scale”, which is a compression of the grain during the milling process that can cause the stain to float or run off without absorption. Clean the surface with an oxygenated bleach.