First, a word about paint
Peeling is rarely caused by bad paint. The same paint that peels in some spots will stick tenaciously for 40 to 50 years in other spots.
Latex paints are the industry standard for most exterior house painting, and they continue to improve. Long-term testing has shown that 100-percent acrylic latex lasts longest. Still, a few pros still like oil-based paints because they carry a slightly better sheen and brush out more smoothly.
Finally, a caution: Scraping and sanding paint that contains lead can release hazardous lead dust. If your home was built before 1979, (lead in paint was banned in 1978), contact your local health department for detection and safe handling instructions.
One secret to paint longevity
Most paint dealers blame paint peeling problems on poor prep work. However, even the best routine of cleaning, scraping, sanding and priming won't stop paint from peeling if water gets into the wood. For this reason, inspect and repair peeling areas annually.
Sure, this sounds like another chore. But it pays off in three ways. First, annual maintenance stops localized peeling and keeps it from spreading. Your house will look better, and you'll protect the vulnerable raw wood from rot.
Second, the cost of whole-house repainting will be lower when that time comes, because you've kept the exterior in good condition. It won't require much prep time.
And third, the paint that's in good condition on your home will last longer, because with longer painting cycles it won't build up as fast. Thicker paint loses its flexibility and cracks more easily, eventually leading to a condition called “alligatoring” (Fig. A). When the paint gets that bad, you'll have the huge job of stripping it all off and starting over.
If you don't have enough time to do the annual maintenance yourself, you'll still come out ahead if you hire a pro for a day or two every year to fix the problems.
Figure A: Peeling Paint
Peeling paint is usually caused by moisture getting into the wood and breaking the paint's bond with the wood fiber. Most moisture comes from outside, but sometimes it collects in the wall and works its way outward.
A paint job is doomed when water gets into the wood. Wet wood causes paint to peel. The process goes like this: Moisture from high humidity or rain works its way into the wood and causes the fibers to swell. The expanding fibers stretch the paint film until it cracks. Once the film is cracked, more water from rain, dew and snow (and lawn sprinklers!) readily seeps through it, soaks the wood and breaks the paint bond.
Water-induced, localized “spot”peeling is easy to identify (Fig. B). Although all joints are vulnerable, wood joints at window and door sills are particularly susceptible. Water sits in the corners, is slow to dry and eventually breaks through the paint barrier. The end grain of window and door frames, once exposed, sucks up water like a sponge, swells and further cracks the paint, making the situation worse. Peeling at wood joints could largely be prevented by sealing the end grain with primer. But carpenters rarely do it. And contractors don't want to bring in painters at this stage because of the cost. But it's worth the effort if you're the one installing the trim.
Otherwise, you're left with the second-best solution: to rely on flexible caulk to seal joints.
Sills and other horizontal surfaces peel because water runoff is slow and in the meantime can work its way through any crack in the paint. Heat from sunlight also drives the moisture into the wood. Scrape, sand and recoat these highly vulnerable areas as soon as peeling begins. Some meticulous painters reduce the problem by applying a water-repellent preservative or thinned, boiled linseed oil to the bare wood to slow moisture penetration. Then they let the wood dry for at least five days before priming it. When paint begins to peel off your soffits (Fig. B), one likely culprit is a clogged or leaky gutter. The best prevention is to clean your gutters twice a year. Many veteran painters recommend removing the gutters entirely and eliminating both the peeling paint and the cleaning chore at the same time. Other painters disagree, because gutters keep runoff from splashing up, soaking and ruining the paint on the lower siding. Despite the cleaning chore, we generally recommend gutters because they protect the foundation from water damage.
If gutters aren't the problem, check for a roof leak. Go up in the attic with a flashlight and examine the underside of the roof for water stains. Fix the roof quickly so that rot won't damage the boards to the extent that they won't hold paint. Poor attic ventilation can also cause soffits to peel. Soffit and roof vents reduce attic humidity, which otherwise will dampen the soffits and lift the paint. (For more on the subject, see Improve Attic Ventilation: Introduction)
Even with gutters, splashing rain often soaks siding boards and trim near the ground. Eventually, they'll absorb water and peel. There's no perfect cure. Maintain your gutters, keep bushes trimmed back from the house to promote drying, and fix peeling areas as soon as they appear. Wood composites like hardboard siding and plywood are especially vulnerable. Keep edges well sealed. Once they swell, they're almost impossible to reseal.
Spot peeling can also occur as a result of moisture from the inside (Fig. A), but the signs aren't always obvious. Usually the cause is high humidity in a room like a bathroom, coupled with air leaks through the wall or the absence of a vapor barrier. Look for tip-offs such as paint that peels around only one window, or that peels from siding in a pattern that outlines a single room. If the problem occurs outside a bathroom or kitchen, you can usually solve it by running a ventilating fan regularly, plugging air leaks around windows with caulk, and repainting interior walls.
Figure B: Trouble Spots
Exterior paint peels where the wood is most likely to get soaked. The most vulnerable spots include exposed horizontal surfaces, joints, wood close to the ground, and soffits.
Bad painting conditions
Although it's less common, peeling paint can be caused by bad painting conditions. Pros in particular, pressured by home purchase closings, tight schedules and bad weather, can get caught working in less-than-ideal painting weather. One veteran painter recounts an occasion when his crew completed the prep work late in the fall, but freezing weather moved in before they could paint. Two weeks later in a surprise warm spell with 50- and 60-degree afternoons, they got the job done. But in the spring, large patches of paint fell off the north side of the house. Thinking back, they could only guess that the north side simply never warmed up enough. And they had a lot of time to think as they rescraped and sanded the entire north side!
Such adhesion problems don't often occur (Fig. C), but when they do, causes are difficult to diagnose. If you're puzzled, don't hesitate to ask the manager of your local paint store or a pro for advice.
Here are the most common causes of adhesion problems:
- Skipping the cleaning step, so that dirt, mildew and chalking cause poor adhesion.
- Painting in temperatures below 50 degrees F. Sure, you might get lucky. Some paints are formulated for cool weather. But slower drying times and complications from dew and frost are all tricky variables to manage. Also, avoid painting too early in the spring. The air temperature might be high enough, but the temperature of the wood surface may still be too low.
- Rushing curing times. Paint sometimes feels dry to the touch before it has hardened. A second coat applied too soon can soften the first and create a weaker bond. The labels on paint cans specify drying times, but humid or cool conditions can lengthen them.
- Painting wet wood. Latex paints, being water-based, can tolerate mild wood dampness and still adhere. But unless you have a moisture meter, you won't know the actual moisture content. Why risk major league failure? Wait until the wood looks and feels dry.
- Using spray equipment without back-brushing. Most pros use spray equipment for speed. But sprayers can't ensure a good bond, so a second painter should always follow closely behind the sprayer to brush the paint into the surface. You also need to back-brush if you're painting with a roller.
- Running afoul of unexpected hazards. Insects, automatic lawn sprinkler systems, direct hot sunlight, high winds and other local conditions can suddenly pop up and ruin a nice job.
- Choosing the wrong paint. Generally on the exterior, you can safely cover oil paint with latex, but not vice versa. In addition, stick with the same brand of primers and topcoats, because some primers and topcoats aren't compatible. As insurance against future paint problems, ask the paint dealer to write paint specifications for your project, including the brand and type of paint. Then you'll have a record should the topcoat peel from the one below.
Figure C: Paint Separation
The problem of one layer of paint separating from another is called intercoat adhesion failure. It's often difficult to diagnose. Common causes include painting when it's too hot, too cold or too wet, a failure to clean the old surface, choosing the wrong type of paint or bad paint and applying too many coats too fast.
Special peeling problems
If individual siding or trim boards peel on your home while others nearby don't, examine their grain pattern. Some types of wood won't hold paint well. Wood used for siding and trim usually has a vertical grain pattern; that is, the tree growth lines in the boards line up close together (Fig. D). Moisture won't affect wood cut this way as much as it does flat-sawn boards, that is, boards with wide grain lines. The surface of flatsawn wood moves so much that paint will crack and peel after only a few years if it's put in an exposed spot. (Plywood also has wide grain lines and won't hold paint either.)
Replace peeling flat-sawn boards with boards that have vertical grain, or sand all the paint off and coat them with a paintable water repellent before priming and repainting. The water repellent might stabilize them enough so that they hold paint longer.
Use a primer over bare wood. Some folks don't, or substitute a thinned paint for the primer. That's not good enough when you want a high-quality job. Primers are specially formulated to seal and adhere to bare wood and other materials. They aren't cheap paint. A good one should cost almost as much as a topcoat.
Finally, always read the manufacturer's instructions for painting cementboard, preprimed hardboard and other wood composite products. The primers on many of them were designed to protect against moisture while the products were being handled and shipped. The primers aren't always a suitable base for paint. You might have to prime the products again.
Figure D: Wood Grain Problems
Paint usually won't adhere long to boards with a wide grain pattern if they're exposed to rain. They expand and contract too much with humidity changes. Vertical grain wood holds paint better because it moves less.