How weather affects paint

If it isn’t already on your pre-job checklist, add this item: Check weather forecast. Why? Because the weather conditions under which you apply exterior latex paint can have a dramatic impact on performance...initial and long term.

You need, of course, to plan around predicted rain for obvious reasons. And most painters are familiar with the guidelines about painting after rain; namely, the capability of latex paints to be applied before the surface has dried 100%, and the need to apply oil-based and alkyd coatings only after the surface has thoroughly dried.

There are, however, a number of other weather-related factors that also must be taken into account, including temperature and drying time.

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

For example, if the 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 vice versa.

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

Remember, too, that the minimum recommended temperature refers not just to the air temperature, but also the temperature of the surface being painted. This is particularly important for the north side of a home or building, which gets less sun than other sides. The surfaces there may be colder than the air temperature, particularly early in the day.

Failure to follow these guidelines can cut years off the life of a paint job. Incomplete film formation, for example, can result in a paint film that fails by cracking or flaking in just a few years or less.