How To Seal Wood For Outdoor Use
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.
When wood is exposed to the sunlight the ultraviolet rays can damage the wood fibers over time, causing the surface to turn gray. The most direct way to renew the appearance is to sand or pressure-wash the surface. However, sanding can be very difficult and time-consuming, and pressure-washing can remove the gray but cause the surface to fuzz or splinter, posing a “touch” hazard for areas such as deck surfaces where people may walk barefoot. When using a power-washer, limit your pressure to no more than 1,000 or 1,200 PSI.
Some individual board may be heavily weathered. Replace them completely, clean the existing boards, then stain them all to match.
Wood with Old Stain
In general it’s best to remove old stain before applying new stain, especially if changing products or colors. Old stain will usually show through the new stain, leaving blotchy spots in the finish. “Film-forming” stains must be completely removed before applying a penetrating stain. However, if you plan to stick with the same color and type of stain (and your deck is in decent condition) then you can probably get by with a thorough cleaning and a fresh maintenance coat. As always, refer to the manufacturer’s recommendations for best practices.
If there is a buildup of old stains on the deck, then you may need to remove it with something stronger than an oxygen bleach cleaner. Stain strippers are more caustic, but they remove most weathered stains in a single application. Small, stubborn spots of stain can then be removed with a hand sander once the deck has dried.
An often overlooked step for outdoor staining projects is the application of a wood brightener. These chemical agents open the woodgrain to improve penetration of the stain and help restore the appearance of weathered wood to a like-new condition. The product can simply be sprayed onto the wood surface, given a few minutes to work its “magic” and then rinsed off, requiring very little labor.
After using any chemical treatment to clean and prepare the wood, use plenty of water to completely remove all traces of the products—and then allow it to dry prior to stain or sealer application.
Always thoroughly mix the stain to evenly blend the solids and ensure a consistent color tone throughout the project.
For both horizontal and vertical structures, brushing is the best method for stain/sealer application because the bristles push the product evenly into the woodgrain to increase absorption. Using a sprayer or roller can apply the stain more quickly, but both of these methods leave much of the stain on the surface without adequate penetration into the pores. This superficial product can wear away unevenly, so you should still back-brush it into the woodgrain for the most consistent appearance and best protection.
For more info contact, Armstrong Clark Wood Stains armclark.com