Bad paint jobs can happen to good people with old houses — even when they listen faithfully to everything the paint salesman tells them. The reason is a surprising revelation about the compatibility of oil and latex paints.
About one in 10 paint job goes awry, says David Chupka, a technical manager for the Sherwin Williams Co. Often it’s because of cutting corners — not sanding, not scrubbing, painting just before a storm,ignoring long-term moisture penetration. But people who own old homes can do everything they’re told by paint salesmen and follow labels devotedly and still wind up with paint that peels. If they’ve hired someone to do the work, at prices that can rival the cost of a new car, peeling paint can begin to look like paper dollars floating off with each breeze.
The problem can occur when an old house with multiple layers of oil-base paint is coated with a modern water-base paint, says Feist who headed the federal government’s house paint research program for 20 years. “The homeowners decide to upgrade and put on a good latex paint. But that last coat of a new type of paint can be sufficient to cause catastrophic failure, often right down to bare wood. ”
There are two kinds of house paint: oil (also called alkyd because of the alcohols and acids used to make a synthetic oil) and so-called latex (which, it turns out, has no rubber in it). Both consist of three main components: a pigment, a binder that glues the pigment to a surface as the paint dries and a solvent that makes the mixture loose enough to brush on. Oil paint forms a tough plastic film as the binder reacts with oxygen in the air. The binder can be a natural oil, such as linseed squeezed out of flaxseed, or oil modified with alkyds. Latex paint forms a flexible film as water evaporates and the once-floating spheres of binder and pigment move closer together and fuse. Latex paint was inverted at the end of World War II using synthetic rubber as the binder. Today the binder is most often a pure acrylic, a vinyl-acrylic or a vinyl acetate. The critical difference between oil and latex paints is that they do not cure in the same way. Oil paint never stops curing. As it ages, it continues to oxidize, becoming more and more brittle. Latex cures in about two weeks and stays pliable. Oil paint generally adheres better to problem surfaces because the oils are small enough to seep into the wood or microscopic openings in old, even chalky paint. The resins in latex paint are generally too big to seep into anything. But that can be advantageous. The gaps between the larger particles in latex paint allow water vapor to pass through. This makes latex less likely to peel from homes with excessive interior moisture.
New Paint and Old Houses
latex outperforms oil and the expensive all-acrylic latex works better than less expensive latex with vinyl acrylics. “The most expensive paint,” Gozdan says, “is the cheapest in the long run.” Rohm & Haas has a vested interest in this position: Paint gave it a postwar market for acrylics, which had been going by the ton into Plexiglas airplane windows. Still, one of the few independent paint research centers in the country, the U.S. government’s Forest Products Laboratory in Madison, Wisconsin, agrees. The lab compared oil and latex paints on its own test fences strung across a windy hillside. “We have twenty-year-old latex that looks as good as if it were new,” says chemist Mark T. Knaebe. The side of a typical house, he says, should get more protection from two coats of latex over a primer coat than it would from two coats of modern oil paint over a primer. No wonder paint salespeople tout the benefits of latex. But what many don’t realize is that all the tests that find latex to be superior have been done by painting over bare wood clapboards or over wood that had only one or two coats of old paint. No one has tested what works best over many layers of old paint. Most houses built before 1950, as well as many newer ones, are covered with multiple layers of oil paint. When Gozdan is asked how this might affect the institute’s recommendations, his answer is surprising: “I would never use latex over multiple coats of oil paint. You stand a chance of peeling off all the paint if you switch.” Latex paint can literally pull old oil paint off the house, he says. “I’ve seen houses where the paint has come off in four-by-eight-foot sheets.”
When a flexible layer of latex bonds on top of brittle oil paint the old paint becomes a thin rope in a tug-of-war. As sunshine hits the wall, the wood and the latex can expand. But the oil paint in the middle is brittle. Now pulled with double force, it either cracks or loosens its grip on the wood underneath. “The latex tends to accelerate the paint loss,” says Carl Minchew, director of technical services for Benjamin Moore & Co. Gary Barrett, director of technical services for the Painting & Decorating Contractors of America, says the stress on the old oil paint is greatest during the few weeks it takes latex to cure, although the results may take months or years to become fully evident. “It’s the shrink factor of latex,” Barrett says. “It has to coalesce, or it can’t cure.” The force of this effect varies. Often houses with layers of old oil paint can be successfully covered with modern latex. But when it doesn’t work, the results may be disastrous. “It’s very unpredictable,” says John G. Stauffer, director of the Paint Quality Institute. To be safe, Gozdan and Stauffer say, people who have houses with more than five layers of oil paint are best to stick with oil.
Many others in the industry are not so cautious. Homeowners can safely recoat even many layers of oil paint with latex as long as the old paint is adhering well and is in good shape, says David Maurer, manager of product development and color delivery for the Gildden Co., which sold the first latex paint in 1948. “If, categorically, latex going over oil was a problem, I don’t think we’d have any latex paint, or we’d have latex limited to new construction, and that’s certainly not the case, ” he says. “But I will say that if you have a lost of loosely adhering — and it is common on older houses to see oil paints fractured right down to the surface — latex paints can hasten the demise by putting pressure on the poor oil paint. I think oil paint makes a better Band-Aid.”
Oil paint is most likely to crack, the service says, when it is more than a sixteenth of an inch thick, the equivalent of 16 to 30 coats. Homeowners who stick with oil may find the paint thicker and not as durable as it once was. Many manufacturers have changed their alkyd formulas to meet clean-air rules in effect in six states. But even if similar rules take effect nationwide next year, as expected, it still will be possible — and legal — to buy oil paint the way it used to be made. Oddly enough, manufacturers can continue to sell high-solvent paint simply by relabeling it “quick-dry enamel,” “industrial maintenance coating” or “marine paint.” Manufacturers can also sell high-solvent oil paint by the quart, although buying it this way instead of by the gallon typically doubles the cost.
Eventually, every house painted with oil will peel, says Minchew of Benjamin Moore, “because that’s what oil paints do. They continue to oxidize and get brittle.” Once a house reaches this point, he says, owners have two choices for dealing with peeling paint: first they can scrape off all the peeling paint, prime the bare spots, and repaint with latex. Areas that weren’t scraped bare will then peel, and the owners can repeat the process until eventually they have a house entirely covered in latex paint that sticks. Second, they can choose a faster, less ugly method: Strip the house down to bare wood and start over, either with latex paint or a semi-transparent oil stain.
Six Signs of Paint Failure
When paint isn’t the problem Old age doesn’t always deserve the blame when paint cracks and peels. When moisture gets behind siding, it can literally push paint off the front. Read the existing paint for clues:
Bulges or flakes at the top of a wall point to gutter or roof leaks; they should be found and fixed.
Paint peeling on a wall next to a bathroom means condensation on the back of siding is being drawn through the wood when sunshine warms the wall. Install an exhaust fan vented to the outside.
If window trim alone is peeling, pry off trim boards and plug gaps around the frame with a low-expansion foam sealant.
If an entire wall is peeling, the siding may need better ventilation. One solution is to slip eighth-of-an-inch thick wedges under the lower edge of each board or shingle. One paint company sells plastic snap-off devices for this purpose. Do not caulk the bottom edge of clapboards.
These excerpts first Published by This Old House / by Jeanne Huber
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