Failure of Preprimed and Finger-Jointed Wood
One of the most common exterior paint failures we see is on painted wood surfaces, especially wooden window sills and door frames. Many times these failures are related to the loss of adhesion of a factory applied primer. Most primers used for preprimed windows and door frames are fast-drying, brittle, lacquer-based products, that trap moisture in the wood. This trapped moisture contributes to the growth of various lichens and fungi which continually thrive on the wood just beneath the coating surface, causing the primer to peel, along with the topcoat. The failure of coatings on preprimed wood trim and windows is widespread, yet preventable. Many times, factory applied primer covers "finger-jointed" wood that is prone to failure and rotting. Exterior wood primers must be breathable and flexible. For new construction and remodeling, we strongly encourage using a premium exterior Acrylic Primer for all new exterior wood. We discourage the use of factory preprimed wood for exterior service.
Finger jointed material comes in several grades: clear vertical grain all-heart; clear vertical grain (sapwood present); clear flat grain all-heart or clear flat grain (with sapwood). The best choice of course is clear, vertical grain all-heartwood.
Only heartwood is naturally resistant to decay. And vertical grain material holds paint much better than flat grain material because it is more stable. It shrinks and swells less across its face and has less grain-raising than flat-sawn lumber. As wood moves (shrinks and swells) under a coating of paint, it stresses the bond between the paint and the wood. Ultimately the film of paint is sheared from the surface of the wood. Builders often think that they are going to get vertical-grain performance form flat-grain material as long as it is clear. This is just not realistic. It is getting harder and harder to get vertical-grain material because the size of available trees are getting smaller, but specify vertical grain whenever you can get it.
Finger jointed trim is not without its problems. It makes many builders nervous. Steve Ferrari, Project Manger, Kohl Construction in Hadley, MA is a typical example. Kohl construction is a progressive-minded company that readily embraces new technology. Kohl uses all the latest products in their high-end, high-quality homes. But Ferrari is quick to say, "We are reluctant to use a finger jointed product outdoors." Builders like Ferrari balk because of horror stories they hear about finger-jointed trim. Stories that tell of finger jointed brick molding literally falling apart after a few years of exposure. And other stories where finger joints have telegraphed through the finish coating of paint.
For new construction and remodeling, we strongly encourage using exterior Acrylic primer like Coronado 410-11 or similar product for most new exterior wood trim and windows. Not all acrylic primers are created equal. You need super resin content, but you need breathability. Only premium specialized Acrylic Primer will give you lasting protection under your paint job. Because if the primer ever fails, all the paint that will ever be applied will eventually fail along with the primer, no matter how good the topcoat. Don't skimp on exterior coatings.
We discourage the use of factory preprimed wood for exterior service. We hope window and door manufacturers will soon change to superior performing, flexible, breathable Acrylic primer and coating systems. All windows and trim must be properly caulked.