From wood finishes expert Bill Feist: Most pressure-treated wood sold in lumberyards is treated with CCA (chromated copper arsenate). Although this type of pressure-treated wood is paintable, be aware that painting is possible only when the wood has been cleaned (using soapy water and a stiff bristle brush, followed by a clear water rinse), and allowed to dry thoroughly. Getting the wood dry can sometimes be a problem, because treated wood is often sold very wet from the treating process. Depending on the climate and drying conditions, it may be necessary to dry the wood for several weeks before painting.
An exterior all-acrylic latex house paint would be the best choice for painting pressure-treated wood. Exterior acrylic latex house paints can normally be used on many different substrates — aluminum, galvanized steel, masonry, concrete, brick — as well as pressure-treated wood and fiber-cement siding. However, always check the label on the paint can to be sure it is recommended for use on wood products.
If possible, find a manufacturer who also has an acrylic latex primer. The combination of latex primer and topcoat has been shown to give the best overall paint performance on treated wood. I would not use oil-based paint, which does not perform well on pressure-treated wood.
Pressure-treated wood may not be the best choice for exterior trim, since most pressure-treated wood is southern yellow pine, a species that is not particularly good at holding paint. Southern yellow pine, whether or not it is pressure-treated, does not hold paint as well as western red cedar. Since most pressure-treated wood has knots and other defects, any lumber used for exterior trim would need to be carefully selected to find boards that are as clear as possible. Although some lumberyards do sell premium grades of pressure-treated wood for exterior trim, this grade may be difficult to find.
Q. How should paints and stains perform on pressure-treated wood? I haven't had a lot of luck painting the PT wood decks I build: The paint seems to fail after only two or three years. Is this because pressure-treated lumber is so wet, or do the chemicals used to treat the wood cause the paint to come off?
A. William Feist responds: It's not surprising that the paint on your pressure-treated wood decks is failing in two or three years. Normally, decks should not be painted, but should be finished regularly (annually or biannually) with a penetrating semitransparent stain or a penetrating clear finish (especially those finishes designed for use on decks). Paints and solid-color stains simply cannot hold up to the severe exposure of a deck surface. As small cracks develop on the deck's surface from exposure to sun and water, water passes through and the paint soon peels. Penetrating finishes cannot fail in this way because they do not form a film, so they are much more suitable for decks. In addition, the wood species used for pressure treating is usually southern pine, which does not hold paint well because it tends to expand and contract a lot.
Studies at the U.S. Forest Products Laboratory in Madison, Wis., have shown that semitransparent stains and clear finishes will actually last longer on pressure-treated wood (CCA, or chromated copper arsenate). This is because the chromium in the treatment protects the wood surface from ultraviolet degradation. Paints and solid-color stains will perform well on pressure-treated wood that is used in an upright position (on fences, for instance), but only when the wood has been cleaned and is thoroughly dry before painting.
William Feist is a consultant and teacher on wood weathering and exterior wood finishing. He was a research chemist at the Forest Products Lab in Madison, Wis., for 30 years.
From Scott Paints
Allow pressure treated lumber to age at least 90 days or preferably six months before painting. This allows excessive protective chemicals to ooze out of the surface, lowering the moisture level to improve paint adhesion.
If the new pressure treated lumber shows any signs of mold, mildew, or algae growth, pressure clean the surface just as you would for prepping for any paint job, using a dilute solution of chlorine and water. Surprisingly, mildew grows quite well on unprotected, pressure treated lumber and it must be removed before painting.
It is recommend to paint with an appropriate primer and topcoat system. The recommended system for best durability is to prime with one coat of a 100% acrylic primer and then top coating with a 100% acrylic flat or eggshell. The use of vinyl acrylic, acrylic blends, or low quality, non-100% acrylic primers and paints is specifically not recommended.
We do not recommend clear waterproof sealers on pressure treated lumber, as the nature of the lumber eliminates the ability of these sealers to penetrate properly. Sifting on the surface, these sealers can actually attract dirt, mold, mildew and algae causing more damage than you would get if you just left it alone. Staining pressure treated lumber is also not recommended, again due to the inability of stains to properly penetrate into the lumber. It is our experience that a proper primer and topcoat of 100% acrylic premium paints has the best chance of adhering and withstanding the excessive moisture and shrinkage of pressure treated lumber.
"The best way to determine if pressure treated wood is dried and is ready to accept a coating, is with a moisture meter. A 12-15% is considered good. A six months drying period is normal for any wood above a 15% reading. If you are uncertain about the moisture content, sawing a small section off of a board will expose what's inside. If there is moisture left on the saw blade, there is too much to coat."
Wolman Wood Care Products
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