Predicting exterior paint performance | Pro Construction Guide

Predicting exterior paint performance

Predicting exterior paint performance

Weather conditions can have a dramatic impact on paint performance, from the application of each brush stroke to how the paint looks a few years down the line.

To predict exterior paint performance, look to the weather and to the chemical make-up of the product.

The binder in latex paint consists of microscopic particles of polymer that need to fuse together when paint dries to form a tough, continuous film. Improper weather conditions can interfere with this process.

If paint is applied at too low of a temperature, the binder particles may become too hard to fuse into a continuous, durable film. This is because binder particles are thermoplastic, meaning they tend to get harder at lower temperatures, and softer at higher temperatures.

This is the main reason why paint manufacturers specify a minimum application temperature (typically 50°F) for latex products, and why it is essential to only apply the paint at or above the minimum temperature. Ideally, you should not apply the paint unless the temperature is predicted to stay above that minimum for 36 hours.

Surface temperature

The minimum recommended also applies to the surface being painted. The north side of a building, for instance, gets less sun than other sides. The surfaces there may be colder than the air temperature, particularly early in the day. If a surface temperature is too cold, the paint film may not properly form, and may fail by cracking or flaking in just a few years or less.

Exterior paint drying times

Film formation and durability also can be affected if latex paint dries too quickly. That’s because fusion or coalescence of the latex binder takes some time to occur properly. Very quick drying can reduce the mobility of the particles before the film is adequately formed. To prevent exterior latex paint from drying too quickly, try to avoid painting when one or more of these conditions is present:

  • Temperatures over 95°F.
  • Direct sunshine. Work your way around the house so you are always painting in the shade.
  • Windy weather and/or relative humidity below 20 percent.
    Source: The Rohm and Haas Paint Quality Institute

Prevent surfactant leaching

When streaks, water-spotting or weeping appear in a paint job, it may be due to surfactant leaching. Exterior latex paints contain some waterborne ingredients, such as glycols, surfactants and thickeners. All of these eventually come out of the paint film as it weathers, usually over the first several weeks of exposure. If the paint is applied in cool, humid conditions that retard drying, a large proportion of these ingredients can migrate to the surface of the paint as it dries, or shortly thereafter. They typically appear as shiny streaks or blotches.

To minimize the possibility of surfactant leaching, avoid painting late in the day, especially in the spring and fall when cool, damp conditions are expected in the evening or overnight. Also avoid painting just before it rains.

The perfect day to apply exterior latex paint

  • Temperature: Between 60°F and 85°F
  • Relative Humidity: Between 20% and 80%
  • Wind: Little or no wind Sky: Slightly overcast (no direct sun)

Source: The Rohm and Haas Paint Quality Institute

 


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