Exterior Wood Primers
When it comes to exterior wood, using a primer is crucial. A good primer, whether it's acrylic, acrylic/latex, or alkyd-based, allows paint to adhere more tightly to the surface. Primer also improves the topcoat's ability to resist surface moisture.
by Mike Dawson
that painting contractors apply. But if you don't prime bare wood properly, you could end up with a very dissatisfied customer.
"Primers, as mundane and ordinary as they may seem, are the key to the final outcome of any paint job," says Steve Revnew, director of architectural marketing for The Sherwin-Williams Company. Using the right primer in the right situation can make the difference between washed-out color with visible stains or a bright and durable finish.
When it comes to exterior wood, primer is crucial. Without it, paint doesn't stand a chance against the chemistry of bare wood. Most jobs involving sizable expanses of uncoated exterior wood are encountered in new construction. That means siding and trim. The siding is most often cedar lap, or sometimes fir or pine plywood, or even cedar shingle.
Wood, especially cedar, holds a lot of chemicals, some which flow to the surface and right through several coats of exterior paint. A topcoat on bare siding would in short order reveal "cedar bleed" as tree tannins escaped the wood. In addition, escaping moisture is liable to cause a finish paint to peel from most types of wood in a relatively short time if primer is not used first.
The job of the primer is to act as an intermediary between the wood substrate and the topcoat. It should seal, hide, and bind wood fibers to make the surface more uniform. This allows the paint to adhere more tightly to the surface. In other words, when you apply an enamel topcoat to a primed door, you are applying the paint to the primer, not to the wood.
A good primer also improves the topcoat's ability to resist surface moisture and the resulting mildew, a feature that is key in the Pacific Northwest and the Southeast.
There are just about as many primers to choose from as there are substrates to coat. "Knowing which kind of primer to use is crucial to the success of any job," Revnew says.
All primers are made to penetrate and bond. But some can do more, such as hide colors of previous coatings, while others might hide less but do better at blocking emerging stains. The type of surface and the external environment dictate which type of primer to use.
For example, Revnew says, if the enameled door receives the wrong type of primer (one with high hiding qualities but lower adhesion), the primer could fail, taking the enamel finish with it.
There are primers for plastics, concretes, drywall, old paint and any other surface you might encounter. However, the primer selection narrows somewhat when you're looking for something designed specifically for exterior wood. The first breakdown of choices falls into the two basic categories that all painters know — cleanup with soap and water or with solvent.
However, a closer look shows that it's more complicated. And the wide variety of formulas based on linseed oil, alkyd, and acrylic ought to quash anyone's idea that primers are nothing more than low-grade paints.
Acrylic latex products are gaining market share in primers just as they are in finish coatings. Matt Crawford, architectural services representative at Kelly-Moore Paints, says that acrylic technology has made such improvements in the water-based coatings that these primers have "made for better performance and service life compared to oil and alkyd wood primers." Once applicators gain confidence in these acrylic products, they tend to stick with them, he says.
Acrylic sealing primers, usually 100 percent acrylic, hide stains and seal tannins. Some acrylics seal out moisture and mildew or block odors as well. "These primers are ideal for delivering the best appearance of flats and enamels on doors, trim and exterior siding," according to Revnew.
The primer market in some regions of the U.S. has been altered by the introduction of VOC regulations. For example, in many parts of California, the VOC limit on coatings has eliminated many oil- or alkyd-based wood primers unless formulated specifically to block tannin staining.
That means alkyds are acceptable for new cedar and redwood siding, but not, for example, previously painted interior surfaces. Even so, California-based Kelly-Moore recommends California customers use its Acry-Shield line of primers. This is a 100 percent acrylic line of primers that offer stain blocking. They are recommended for exterior wood products found in California — cedar and redwood — as well as the cementitious materials common to the region.
In states without VOC restrictions, Kelly-Moore recommends its alkyd-based primer, Weather Shield.
On the other end of the spectrum, some leading manufacturers still make what could be described as an old-fashioned oil-based primer made with linseed oil. The high-performance brands are enhanced with alkyd to speed drying and improve color retention and flexibility.
One of those manufacturers is California Paints, located in Andover, Mass., which modifies its high performance Fres~Coat Trouble-Shooter with alkyd and barium metaborate. Ron Boyajian, product marketing manager for California Paints, describes this linseed oil-based primer as a premium high-performance product.
It has one shortcoming, however. The recoat time for this primer is 48 hours. This puts a pinch on a contractor's productivity, especially if scaffolds need to be moved to the next wall while the first coat dries.
|Surfaces that show flaking, cracking or blistering should be carefully scraped, wire-brushed or sanded down to bare wood before priming for the finish coat.
To remedy this problem, the company took the same product and modified it with higher volumes of alkyd to produce Trouble-Shooter Fast Drying Alkyd Primer, which has a recoat time of just four hours. Boyajian says the company's engineers were able to create the fast-drying formula without compromising the performance of the oil-based primer.
For those who prefer soap-and-water cleanup, California Paints offers Troubleshooter 100% Acrylic Exterior Primer. It also offers a lower priced line of oil-based and acrylic under the company's Pacific label.
Most other primers for exterior wood are somewhere in the middle, either an alkyd base, a water base, or an oil base with alkyd and other additives. Most manufacturers offer at least one acrylic with alkyd.
Benjamin Moore offers a line of exterior wood primers, in latex and oil/alkyd combinations. In latex, it sells Fresh Start All Purpose 100% Acrylic Primer 023, and Moorcraft Super Spec Latex Exterior Primer 169.
In oil/alkyd, Benjamin Moore makes a Fresh Start All Purpose 024, a Fresh Start Fast Dry 094, a Fresh Start Moorwhite Penetrating Primer 100, as well as the Moorcraft Super Spec 176 and the Benjamin Moore Alkyd Primer 366.
Pittsburg Paints offers its Seal Grip line of primers in both acrylic and synthetic formulas, and advertises one-hour recoat times. Its sister company, Olympic Paints and Stains, markets Deck Primer, and alkyd/oil sealer intended for use on exterior wood prior to staining with solid colors.
The significant pros and cons of these formulas still come down to cleanup. Even the producers who are committed solely to acrylic products had nothing negative to say about the performance of oil-based primers. Nor do they debate whether to use a latex finish coat over an oil-based primer. Even the most committed latex marketers agreed that there is nothing wrong with applying latex over an oil-based primer.
Boyajian, of California Paints, which emphasizes linseed and alkyd products, summed it up: "Latex over oil is fine." For that matter, he says, "You can put anything over anything." Other manufacturers agreed.
Even an oil-based finish coat is appropriate for an acrylic primer. But, as one maker of oil-based primer says in a moment of candor, "You would be insane to use an oil-based paint once you've converted to a latex primer."
On the issue of cleanup, some would suggest that when using high-performance, oil-based products, discard the applicator tools at the end of the day.
Of course, that depends on the tool. Most manufacturers prefer their product to be sprayed for the best coverage and penetration. Brushing is also commonly accepted. Oil primers roll on nicely, although Boyajian says he is not a fan of this method unless followed by a brushed coating.
Manufacturers also warned the public to beware of siding that comes primed from a plant. Those plants, in order to stack the lumber in a timely way, are obviously using an inferior primer that dries in a very short time, experts like Boyajian say.
Whenever priming bare exterior wood, keep in mind that this first application will dictate the appearance of the structure for years to come.