Cool Weather Painting
Fall is a great time to finish up outdoor projects, but a lot of homeowners wonder if it's safe to paint in cooler conditions. The short answer is yes...but you'll need to pay closer attention to weather conditions than during the longer days of summer. This is because most paints are formulated to cure best when temperatures are around 70 to 77 degrees F and relative humidity is around 50%. The more temperature and humidity deviate from those optimal conditions, the riskier it is to paint. Here are some quick guidelines to help you determine whether you should pick up a brush, or move on to other projects on your To-Do list.
Temperature. In summer, a good rule of thumb is to plan your painting so that you're always working in the shade, so that surface temperatures don't get too hot. But in cooler months, painting in direct sun is actually a good idea, because this will help boost surface temperatures up into the safe range--around 70-degrees--even when air temperatures are chillier. In general, paints should be applied only when both surface and air temperatures are above 40 degrees; primers generally need temperatures above 50 degrees to dry properly.
Not only should temperatures be above 40 degrees when the paint is applied, but they should remain that way for at least 2 or 3 hours for paint to dry properly. Usually the best window for painting in cooler conditions is between 10 am to 3 pm, but it can be considerably shorter in marginal conditions. Dew is another reason to avoid an early morning start. Watch for signs of moisture on the lawn and make sure it's fully evaporated from the work area before starting.
It's also important that air temperatures don't drop below freezing the first night after paint has been applied, since curing paint can still contain moisture that will crystallize in sub-freezing temperatures, instead of evaporating out into the atmosphere as it's designed to do. (If temperatures do drop, you won't see a problem until the following spring: Moisture will remain hidden in the wood siding over the winter, then migrate out into the paint under a warm spring sun and form blisters.)
Humidity. Relative humidity is harder to measure than temperature, but it plays an equally important role in how well paint cures. The ideal is 50% relative humidity, but curing times are significantly affected when humidity levels exceed 70%. In high-humidity situations, you could see what is called surfactant leaching, brown or white discoloration on the surface of the paint. Minor surfactant leaching can be rinsed off with a hose or can be left to wash away with normal weathering.
Low relative humidity can also be a problem, because the paint surface may dry too quickly and lead to blistering later on. But this is less common in cooler temperatures; more often, the problem is too much wind, which can also dry paint too quickly and deposit dust and other particles on the surface.
Finally, it's probably a good idea to avoid pressure-washing in the fall, since it will take a long time for your siding to dry out. Pros use moisture meters to determine the moisture content of the substrate that they're painting, to avoid blistering later on; a safe level for wood shingles and clapboards is 12% moisture content.