Sealers & Finishes for Pressure-Treated Lumber
Although treated wood is protected against decay and termite attack, the application of a water-repellent sealer to all exposed wood surfaces is recommended upon completion of construction. This sealer will help control surface checking (splitting or cracking) and provide an attractive appearance. Over time, reapplication of a sealer is recommended, perhaps every year or two; follow the manufacturer’s instructions.
Over several months, pressure-treated Southern Pine lumber will weather naturally to an appealing silver-gray color.
Treated Southern Pine lumber will accept a finish similar to untreated material. Most importantly, Southern Pine should be dry before any type of finish is applied.
Following construction, most manufacturers of stains and paints recommend a waiting period – from a week to two months – before applying a finish to treated wood, if the project was built with lumber that was not kiln-dried after treatment (KDAT).
Most paint manufacturers recommend two coats of a good-quality acrylic latex paint for best results on treated Southern Pine. When applying any type of finish, be certain to follow the recommendations of the paint or stain manufacturer.
Fences are fully exposed to the weather, and have at least some parts in contact with soil. Care must be exercised in selecting and applying a finish to obtain a reasonable service life.
Many fences are left to weather naturally. However, if a finish is desired, semi-transparent penetrating stains or water-repellent sealers/preservatives that contain a mildewcide (or otherwise are resistant to mildew) are preferred. These finishes are absorbed into the wood without forming a film; they do not crack or peel. Stains come in a variety of colors and show the wood grain.
If paint is to be used, liberally brush the surface plus all ends and joints with a paintable water-repellent/sealer preservative and allow the surface to dry for at least two warm, sunny days before painting. Apply one coat of a good-quality stain-blocking acrylic latex primer, followed by two topcoats of a good-quality acrylic latex exterior house paint.
When repainting, scrape all loose paint from the wood, then use a stiff bristle brush to remove any remaining loose paint and dirt. Next, brush on a paintable water-repellent sealer/preservative. Apply it liberally to exposed ends of boards or pickets and to all joints. Let the treated wood dry, then apply acrylic latex paint.
Varnish finishes and solid-color stains are not recommended for fences because they will not withstand sun and rain and will require frequent refinishing.
Decks & Porches
Decks and porches present a particularly severe exposure for both the wood and finishes. Most wood members are in a horizontal or flat position. These horizontal surfaces, especially in decks, are often exposed to the direct rays of the sun and tend to collect moisture, so the weathering process is greatly accelerated. As repeated cycles of wetting and drying occur, checks tend to enlarge rapidly into cracks and, along with the end-grain surfaces, begin to retain moisture. The conditions for decay and insect attack caused by the presence of moisture are thereby greatly improved.
Any film-forming finish is likewise subjected to excessive stress because of the continuous shrinking and swelling of the wood that results from changes in its moisture content. Furthermore, the finish is subjected to abrasive wear, particularly in high-traffic areas. By design, porches are somewhat protected, so the conditions are not normally as severe as those with decks; however, the same conditions — moisture, sun, and abrasive wear — are usually present at least periodically.
For fully exposed decks, a water-repellent sealer or a penetrating semi-transparent stain may provide the best finishing solution, even on wood that has been pressure treated with preservatives. Special formulations made specifically for decks are available. These penetrating deck finishes, at least the water-repellent sealers, may have a shorter service life than paint, but they are more easily renewed. For severe exposures, the finish should be renewed annually; spring is usually the best time. Light-colored penetrating stains will also last longer than dark ones on flat surfaces subjected to traffic, because light stains show the least contrast in grain color as wear occurs. The penetrating finishes need to be refinished every 1 to 2 years.
To refinish, a thorough cleaning of the wood with a stiff bristle brush is usually adequate before applying the water-repellent sealer or penetrating stain finish. Paint and solid-color stains, particularly in these applications, are likely to peel. Laborious scraping and sanding before refinishing will usually be required for these finishes. Therefore, paint and solid-color stains are not appropriate for fully exposed decks.
The bright color of the wood on weathered decks can be restored (and surface mold and mildew can be removed) by application of commercial products (labeled as deck cleaners, brighteners, or restorers). These products may remove the weathered wood surface; some care should be exercised not to remove excess wood. Color can also be restored using a liquid household bleach. The bleach is usually diluted with water (1 part bleach, 3 parts water) before it is applied to the deck. The bleach solution should be thoroughly rinsed from the deck with water. If the deck is to be finished after cleaning, allow 1 or 2 days drying time.
Paint may be used successfully on roof-protected porch floors. For optimum results, first treat the wood with a paintable water-repellent sealer/preservative. After the floor dries, a primer and two topcoats of porch and deck enamel should be applied. Porch enamel is especially formulated to resist abrasion and wear. Solid-color stains should never be used on flat surfaces such as decks and porches, because of their low resin content.