How Heat and Humidity Impact Exterior Paint

All exterior paint interacts with its environment during application and over its lifespan. This means that factors like heat and humidity must be understood and accounted for when planning your exterior painting project.

The Challenges of Heat

High painting temperatures can be a challenge, especially if painting in direct sunlight. These factors combine to make paint dry extremely fast, which creates two pitfalls. First, it’s difficult to maintain a wet edge, which results in lap marks in the body of the paint and hat banding around trim areas. Second, the paint dries to the touch so fast it’s tempting to apply a second coat before the first has fully cured. It’s important to follow the instructions on the timeline for the second coat, or you’ll risk the first coat peeling off.

Direct sunlight also impacts the lifespan of the paint, especially on southern-facing surfaces where exposure is greatest. The sun (or reflective surfaces) can cause paint to heat faster than the underlying surface, creating “heat bubbles” that eventually burst and peel. The sun’s ultraviolet rays will also bleach most paints, making patches and repairs more difficult.

The Challenges of Humidity

Humidity during painting slows drying times. In an extreme case, air may be so saturated that the paint will struggle to form an adequate bond and begin to run before drying. This is especially true if painting metal, because the moisture in the air will tend to condense on the metal surface, creating a moisture barrier between the surface and the paint. While precise recommendations vary by area and conditions, if the relative humidity is 85 percent or higher, hold off on painting.

Humidity can also affect exterior paint after the application. It encourages mildew growth, especially on shaded or north-facing surfaces with less sun exposure.

Coping Strategies

The first step in dealing with heat and humidity is selecting the right materials to work with. If you know a surface will be shaded and exposed to damp conditions, consider a mildew resistant paint or a mildewcide additive. For projects with exposure to the sun, you may opt for a paint specially formulated to resist fading. And if your project deals with extreme heat, such as a grill or fire pit, choose a specialty paint designed for this kind of application.

Once you have the right paint, approach your project with forethought. Accept that exterior painting is weather-dependent, and that you may not be able to paint on the day you would like. It sounds simple, but many exterior paint projects end in frustration because painters want to get “one last coat on before the rain hits” only to find a streaky mess once the storm clouds are gone.

Also, take the sun’s path into account. Note what parts of the project are in shade, and how that will change over the course of the day. Time your project so that you get the bulk of the work done before or after the hottest time of day. And see what the weather forecast calls for. Sometimes an overcast day means that you don’t have to worry about the sun at all. It’s always worth doing a little extra planning to avoid a streaky finish.