Eliminating Chalk Deposits from Exterior Surfaces
Why Does Chalking Happen, and What Can You Do About It?
When paint on outdoor surfaces ages, it develops a light but noticeable buildup of powder. This phenomenon is known as chalking. This is a natural process; paint that chalks is much less troublesome than paint that peels.
Drawing a rag, or even your hand, across an exterior surface will usually tell you if chalking has occurred. A light-colored rag reveals chalking on dark-colored paint, and vice-versa. Chalking powder is generally a bit lighter in color than the paint it comes from.
Natural Light and Oxygen: The Usual Culprits
The UV light that’s part of sunlight degrades the chemicals that keep coloring agents attached to paint’s other components. Another natural substance that affects paint chemically is oxygen, which brings about oxidation when it reacts with the binding elements of paint. Both light and oxygen make paint less bright and viscous. After chalking occurs, rain may remove the powder, leaving paint on the outer surface thinner than it was before.
Other Factors, and Preventive Measures
Acrylic paint holds up better than oil paint on outside surfaces because it undergoes oxidation more slowly. Whatever type of paint you choose, its quality — often indicated by its price point — plays a significant role in its resistance to chalking. Less expensive paint tends to include less effective resins — the ingredients in paint that bind (retain) colorants. Once the resins have been oxidized, the pigment quickly becomes susceptible to chalking. An example: Paint that is entirely acrylic, and thus more expensive, lasts longer than latex paint, or a mix of the two.
Getting Rid of Chalking Powder
Before you paint, the powder must go. A pressure washer loaded with a gentle cleaning solution should do the trick. Pressure wash according to proper guidelines, and let the solution stay on the wall for up to 15 minutes before removing it. Rinse at between 1500 and 2000 psi. Exterior surfaces can take up to a full day to dry. If significant chalking has occurred, you may wish to apply an acrylic primer before repainting. Keep in mind that you aren’t likely to get every bit of powder off the wall; don’t be surprised if you still see some here and there after washing.