To prevent paint failure, make sure wood windows are properly flashed, prepped, and painted … and don't forget the caulk

When I lived in New England, I was surrounded by decades-old traditional wood windows that still held paint quite well. But where I live in Seattle, it's not unusual to find expensive, recently manufactured windows being replaced before they're even five years old. I know of a three-year-old house that didn't sell until all of its wood windows were replaced — with vinyl. Modern wood windows seem to have earned a bad reputation as being notoriously difficult to keep intact. What gives?

Why Does Paint Fail?

The sash of a traditional wood window — that is, a window with panes of glass held in place with glazier's points and glazing putty — is essentially composed of four single, dimensionally stable pieces. But a modern window, with insulated glass sandwiched into a wooden frame, can have two or three times as many pieces, which are often smaller and frequently made from finger-jointed wood. This results in a number of joints, edges, and end grains that all move in different directions at different rates, presenting openings for moisture penetration and offering a tricky substrate for paint.

Compounding the problem, modern windows sometimes seem as if they're designed to attract — rather than repel — water. Often, they'll have vertical joints that act as channels, collecting water and transporting it onto the sill. If the window's sill has an inadequate or negative slope, water will pool on it or even be directed toward the house and against the window frame's vertical end grain. Some windows substitute wooden trim for glazing compound. Narrow and dimensionally unstable, these pieces can trap water on their unprimed backsides, bow out, and eventually rot. In other cases, a window manufacturer will substitute an adhesive for glazing compound, but it, too, quickly rots out and forms water-holding channels.

Other manufacturing practices only make these problems worse. Some windows are built with acute angles that look great to the designer but are too sharp to hold paint well. In addition, factory-primed windows are generally primed with an alkyd primer that rapidly oxidizes and dusts up if not top-coated in 10 to 14 days. This dust prevents bonding of future coats of paint to the surface. Sometimes, the window's sash and frame aren't completely primed or painted, leaving bare wood that invites airborne moisture to enter the window. Even unprimed windows present a problem: They're subject to mill glazing, a condition where the cellulose in machined surfaces gets plasticized, removing the "tooth" necessary for paint to adhere properly.

When it comes to top coats, many painters who work with a brush prefer oil-based paint because of its self-leveling properties. Unfortunately, oil-based primers and top coats are too brittle for exterior applications. Modern windows, which are made up of many parts, are vulnerable to temperature extremes, expanding and contracting with changes in the weather. This thermal cycling causes inflexible oil-based paint to crack and eventually fail, which is why you generally see much greater failure on the south sides of houses. In fact, even on the same side of a house, I've seen differences in paint durability between the first-floor windows and the eaves-shaded second-floor windows. In every exterior application, 100 percent acrylic latex paints are superior to oil paints today. This is especially true on modern wood windows, where flexibility is crucial.

Proper Installation Comes First

Wood windows can be doomed by shoddy installation practices. For example, head flashing is so rarely installed these days that I'm actually amazed when I find a window that has it. Without flashing, storm-driven water can get under clapboards or other types of siding and start rotting the top of the frame. I see this over and over again, especially on first-story windows that don't get protected by the second-story eaves.

Flashing is even more important when wood windows are mounted in walls clad with brick, stucco, or other types of nonwood siding. Because the wood window and the siding material expand and contract at different rates, a gap forms that can trap water against the window's wood frame. In every case where I've found an unflashed window in this situation, I've also found rot. Typically, windows that aren't flashed don't get caulked around the edges, either, presenting yet another entry point for water.

Water vapor is also a problem. Even when a window's interior and exterior surfaces are properly sealed with paint or lacquer, often the window's edges never get finished. Especially in a tight house, the gaps around windows and doors are among the few places interior moisture — in the form of water vapor — can escape. Because there's no impermeable membrane (in the form of paint) to prevent it, moisture can be drawn from this vapor path into the bare wood edge of the window, right around the weatherstripping. Sunlight then pulls the moisture through the wood; when it reaches the wood's surface, it sits between the wood and the paint. The result is disbonded paint. Frequently, you'll find the interior paint peeling on these windows as well.

Proper flashing and caulking are the best ways to avoid rot, of course. But another strategy that will prevent — or at least slow — rot is to prime the exterior of the frame all the way around, including the edges, before the window is installed.

Prep Before Painting

Whether a window is newly installed or old and peeling, the key to getting paint to stick is proper preparation. In some cases — for example, old windows that are in really bad shape or new unprimed wood windows — I'll remove the sash from their frames to do this work. If the interior surface is already painted, I'll immediately mask it off to protect it.

On houses with badly peeling windows, it's fair to assume that paint that hasn't peeled yet will peel eventually. Even so, most people cannot afford to have all of their windows stripped to the bare wood simply as a precautionary measure. So I concentrate on the worst windows, which are typically found on the south side of the house. I'll work on them until I can no longer remove any paint, which usually takes about an hour.

To remove paint, I use a combination of ProPrep molding scrapers (Preservation Resource Group, 800/774-7891, www.prginc.com), a palm sander, and putty knives. The fact that most of the failures noted above cause disbonding and failure down to the bare wood makes paint removal somewhat easier.

Once I've thoroughly scraped the window, I use Sherwin-Williams' resin-based spackling (Sherwin-Williams Co., 800/331-7979, www.sherwin-williams.com) to putty any fastener holes made during the manufacturing or installation process. If there is any rot or even serious checking on the sill, I do repairs with epoxy.

Next, I sand the entire surface with a palm sander loaded with 50- or 60-grit sandpaper. This removes any oxidized layers, roughs up the bare wood, and feathers the old paint. The feathering is crucial, as sharp edges from old paint will cut right through new paint, continually expanding the peeling area. (Look at any house with a problem paint job and you'll see that the new failure starts right where the paint was last scraped.) I also round over any acute corners just enough to remove the factory knife edge that guarantees paint failure but not enough to be noticeable from the yard.

Finally, I scuff the exteriors of all unprimed new windows to remove mill glazing. I use a dry trim brush to clean off the dust, then wipe down every surface with a rag lightly dampened with mineral spirits, which quickly evaporates to leave a clean, dry surface ready for paint. (Don't use a water-dampened rag, because it might raise the grain and lift the carefully feathered paint edges.)

Even when I'm planning to spray the top coat, I generally hand-prime bare wood with Sherwin-Williams' A-100 latex primer, particularly when repairs are minor. While many manufacturers claim that their top-of-the-line 100 percent acrylic latex paints can be used without priming, priming at this stage prevents dew from getting under the newly feathered edges and lifting them before I get a chance to paint. Besides, I figure that if these paints will work over bare wood, having a primer coat certainly won't hurt them.