But what about in-between conditions, such as when it is humid? If so, how much humidity is acceptable for you to paint before you risk paint failure.
Few Conditions Are Ever Perfect
This question is important to anyone who lives in a damp climate.
If you stopped painting your house due to humid conditions, you would never finish your project.
Yet a moist surface is one of the main causes of failed paint. So critical is this that fully wet wood siding requires several sunny and/or windy days to dry properly before painting, according to Mark Knaebe, a Chemist at the USDA Forest Products Laboratory.
In rain, dew, or other moisture conditions, the main issue is how damp the surface is, not the air.
Hinges on Water and Solvent Drying Times
Still, this may not necessarily bar you from painting. After all, unless you live in a perfectly arid climate, your air is humid. Air naturally holds moisture.
Knaebe provides insight as to how humidity can affect a painting surface.
It comes down to a race between drying times of water in the paint and the solvents in the paint. Which will dry first?
Water needs to evaporate at a rate as fast or faster than the solvents.
If the water interferes with solvent-drying, then the paint cannot form a hard surface.
When it is too humid, water cannot evaporate and the solvents may evaporate first, causing the paint to cure while still in a water-filled state. You cannot recover from this type of disaster.
This does not just affect water-based paints.
Knaebe notes that oil-based paints are prone to the same conditions.
Optimal vs. Acceptable Humidity Levels
Optimal relative humidity (RH) levels for exterior painting, says Karl Crowder of Crowder Painting from Colorado Springs, Colorado, tend to be in the 40-50% range or lower.
Levels of 70% or greater will "drastically slow drying and curing," but again will not prevent you from painting.
Painting above an RH level of 85% is strongly not recommended. The paint will remain gummy and gel-like until the RH lowers to an acceptable level long enough for the paint to solidify.
However, because the paint has not been allowed to "level" properly, the texture will remain wavy and otherwise unacceptable.
Temperatures Work With Humidity, Too
One tip is to time your painting so that temperatures are on the rise--several hours before the day's peak temperature, which is usually in the late afternoon.
Sherwin-Williams reinforces the idea that you need to build in a head-start by beginning hours before you think it is time to begin:
Even though the temperature was OK at the time of application, the paint can stop coalescing. This permits moisture to get into the uncured paint film allowing certain ingredients to come to the surface when the moisture evaporates, causing surface staining and possible adhesion problems.
Yet if you begin too early in the day, you have to content with accumulated dew on the surface.
Waiting For the Right Season For Raw Wood
Generally, if you are painting a coated surface, you can wait until late morning or about noon, when the dew has burned off of the surface. When the moisture visibly appears to be gone, you can be confident that it truly is gone.
If you are painting raw wood, it is highly porous and will retain moisture within the wood's cellular core--even if it looks and feels dry. But it you wait too long in the day, you run into the problem of dew beginning to form again.
Your window of available painting time shrinks to zero. For situations like this, you have no choice other than to wait for a warmer, drier season.
What About Interior Painting?
Interior painting is a different matter because the climate can be controlled.
For one, the surfaces are protected; unless you have a leak in your home, your walls should not be wet.
More importantly, HVAC (air conditioning or heat) or dehumidifiers running within the house can void the house of excessive moisture.