PAINT CHECK: Paint chemist Stewart O. Williams, Ph. D., inspects test samples at the Rohm and Haas long-term outdoor exposure facility in Pennsylvania.  Continued research has improved paint performance in recent years.
PAINT CHECK: Paint chemist Stewart O. Williams, Ph. D., inspects test samples at the Rohm and Haas long-term outdoor exposure facility in Pennsylvania.  Continued research has improved paint performance in recent years.

Q: I have some homes that need exterior painting, and cold weather is closing in. My painters say they can paint outdoors when it’s as cold as 35 degrees F. Should I be worried?

A: Things are always changing in the world of house paint. Just a decade ago, labels on exterior-paint cans warned not to apply the paint unless temperatures were at least 50 degrees F. But in the past few years, most manufacturers have brought out acrylic latex paints with a much wider application temperature range, allowing painters to work in temperatures as low as 35 degrees F.

Flexible Formulas

What’s changed in paint formulas? Acrylic paints are a carefully crafted mix of a few key ingredients. There’s the pigment, which gives the paint its color and protects the substrate from sunlight. There’s the binder; in acrylic latexes, that’s acrylic, a plastic polymer molecule, which chemists can adjust for hardness, flexibility, or other desired properties. (Cheaper formulas may use vinyl along with the acrylic.) There’s the solvent, or carrier; in latex paints, the solvent is primarily water. There are also surfactants, basically soaps, that lower the surface tension of the liquid paint and (along with other flow agents) affect how smoothly the paint loads up on the brush and flows out, or how readily it sprays and lays down. And there are small amounts of coalescents—nonwater solvents that remain in the fresh paint a little longer as the water dries out, helping the acrylic binder molecules to soften and melt into each other to form a strong, flexible, ­continuous film.

Designing a good mixture is something of an art form, and paint makers invest major dollars in researching and experimenting with different blends. To push the application temperatures lower, formulators have mostly been tweaking the acrylic binders and the coalescent components of the paint, says paint chemist Stewart Williams, Ph.D., technical director of The Paint Quality Institute in Spring House, Pa. The institute is an educational service of chemical company Rohm and Haas, a firm that supplies most of the paint industry with acrylic resins and other ingredients. “To lower the temperature where a film can form, you can go to a softer binder,” explains Williams, “but if you push that too far, you can run into a problem with good hardness development. So part of the solution is to add extra coalescing aids.”

Does it work? There’s no question that the new formulas have opened up a wider comfort zone for cool-weather painting. If temperatures are hovering in the 40s, you can now paint without worrying.

But there’s always the temptation to push your luck. Many ­products get fussy in cold weather, notes Nigel Costolloe, who runs Brookline, Mass., custom painting company Catchlight. “All exterior materials, from epoxies to spackles to caulking to paint—everything is retarded by cold temperature,” says Costolloe. “It can stretch the production schedule out almost infinitely. Here in New England this [past] fall, we had a succession of reasonably mild days followed by pretty cold nights, and then rain every three to four days, and there really wasn’t a window in there to get some exterior painting done for weeks.”

Pushing the Envelope

To keep things moving through the year, says Costolloe, “I would say that just about every painting contractor at this point pushes the envelope. They start painting earlier in the year, and they finish up later in the year. Or at least they keep their fingers crossed that they can finish projects later on. ”

But the closer you get to the edge, the greater the risk. “No question,” says Williams, “there’s more of an opportunity for things to go wrong [when you’re] painting in extreme conditions [rather] than under normal conditions.” South Woodstock, Vt., painter Charles Gilley Jr. has little choice: “If we only painted when the weather was perfect,” he jokes, “we’d paint maybe three days out of the year.” But Gilley, owner of Restoration Painting, says, “If you exercise caution, you’re going to be fine. It’s when you really start pushing that envelope—it’s 36 degrees F, and I’ve got two or three hours, and then all of the sudden it’s due to go down to 15 degrees F tonight—that’s not good. [Then] you’re asking for trouble.”

What kind of trouble? In mild cases, it’s not too bad. “What typically happens is that you get a surfactant leaching on the paint that can usually be washed off without much damage to the actual paint film,” says Costolloe. “It can show up as a chalkiness on dark paint that will require a visit in the spring to resolve. It will certainly hang up closing out a project.”

“You see it if the walls don’t cure enough before the dew hits them,” says Gilley. “The dew brings out the surfactant, and the stuff will start weeping down over the siding like little icicles running down the faces of the clapboards. Eventually the weather will wash it off, but initially it’s an objectionable trait.”

Occasionally the problems are more serious. “You can get into worse trouble by compounding mistakes,” says Costolloe. “Applying a primer, for instance, and applying a top coat and another top coat on that one with insufficient drying time between coats. If you have an overnight minimum of 35 degrees F and daytime temperatures in the mid-40s, you’re still retarding the drying and the curing process. And so trying to achieve the same production rates as you expect in warmer weather just won’t work. You need to extend the drying time, especially when you have shorter days, longer nights, and a closer dew point.”

Multiple coats applied too quickly in cold weather may show bubbling and blistering. “You could end up with a more catastrophic failure where intercoat adhesion would be an issue,” says Costolloe, “and you’d have a paint coating that would need to be removed. That’s worst case.”

Precautionary Measures

What steps should you take before venturing out in the cold with paint? Our experts offer these tips:

  • Make sure the paint is actually rated for 35 degrees F. Many common house paints and stains are still restricted to 50 degrees F and up.

  • Pay special attention to the substrate. “You really want to know the moisture content of your substrate,” says Williams, “and priming is critical.” Check the wall’s temperature with a non-contact infrared thermometer, too, not just the air temperature—both should be above the label-specified minimums.

  • Work midday. Focus on prep work in the early morning and late afternoon, and apply paint between 10 a.m. and 2 p.m., advises Williams, to allow surfaces to warm up and to allow time for curing before dew falls.

  • Follow the sun. “In summer,” says Gilley, “the rule is, don’t paint in the sun—follow the shade around the house. But in cold weather, it’s the opposite. Because we want the paint to set, we do paint in the sun.” What about the north side? “That makes it difficult,” says Gilley.

If it’s just too cold but you really need to paint, there’s always the option of shelter. “You can set up scaffolding, wrap everything in plastic, and heat it,” says Gilley. “But I would never do that. I would just say the heck with it. There’s always next year.” Learn more about markets featured in this article: Boston, MA.