Chalking Paint and the Chalk Test
By Lemoon's Painting
Chalking is the powder left on a painted wall when sunlight breaks down the paint's binder and frees the pigment. You can test for it by rubbing a dark cloth or pressing tape on a sunny wall. Before repainting, the chalk has to be washed off until the test comes back nearly clean, because new paint bonds to the powder rather than the wall.
Run your palm across the south wall of an older house in Chula Vista, Spring Valley or Santee and there is a good chance it will come away colored, as if you had brushed against a chalkboard. That powder has a name, a cause and a consequence for the next paint job. It is one of the most useful things a homeowner can check without any tools.
What is chalking, exactly?
Paint is pigment held in a binder. The pigment gives color and hiding; the binder holds it together and glues it to the wall. Ultraviolet light slowly breaks the binder down at the surface. As it erodes, the pigment particles are left loose on top, and they rub off as a fine powder in the color of the paint.
A little chalking is normal aging. Heavy chalking means the surface of the coating has worn away to the point where it is no longer protecting the wall as it should.
Why does it show on some walls more than others?
Sun exposure. South and west elevations in the inland valleys take the most ultraviolet light and chalk first. A north wall on the same house may hardly chalk at all.
Color. On a deep color, chalk shows as a pale haze, so the wall looks faded and washed out. On a white or light wall it is harder to see but just as present.
The paint itself. Paints with less binder relative to pigment chalk sooner. So does paint applied too thinly, because there is less film to wear through.
Age. A wall that has gone many years without paint will almost always chalk.
How to do the chalk test
You need a dark cloth, or a strip of clear or dark tape, and a few minutes.
- Pick a sunny wall. South or west, at a height you can reach comfortably.
- Rub firmly. Wipe a dark cloth across the wall with moderate pressure, or press a strip of tape onto the wall and smooth it down hard.
- Look. Lift the tape, or look at the cloth.
- Compare. A faint trace is light chalking. An obvious band of color is moderate. A cloth or tape that comes away thickly coated is heavy chalking.
- Repeat on a shaded wall. The difference between the two tells you how much of the wear is sun.
Painters often use a more formal version of the tape test to rate chalking on a scale, but the principle is identical.
If the white material is crusty or fuzzy rather than powdery, or appears near the ground and in patches rather than evenly, it may not be chalk at all. How to tell chalk from efflorescence.
Why chalk has to be washed off
New paint bonds to whatever it touches first. On a chalky wall, that is a layer of loose powder, not the old coating and not the stucco. The new film can look perfect on the day and then lose adhesion well before its time, because it is anchored to dust. Peeling and flaking a few years after a repaint very often trace back to chalk that was painted over.
That is why the power wash is the first step of every exterior repaint, and why it is about more than dirt. How a house is power washed before painting.
How chalk is removed
- Wash. A thorough power wash at a pressure suited to the surface, often with a suitable exterior cleaner and some scrubbing on heavily chalked areas.
- Let the wall dry. A wet wall can hide chalk that is still there.
- Test again. The chalk test is repeated on the washed wall. If the cloth or tape still comes away colored, the wall is washed again.
- Prime where needed. When a wall tests nearly clean but a trace remains, a primer formulated to bind light residual chalk is applied before the finish. Both Dunn-Edwards and Sherwin-Williams make primers and conditioners in this category. Primer is not a substitute for washing; it is what comes after a good wash.
- Then paint.
Do not forget what is below the wall
Chalk does not stay on the wall. Rain and sprinklers rinse it down onto walkways, foundations, brick planters and window sills, where it leaves pale streaks. Those surfaces are usually cleaned at the same time, and any that are being painted are tested the same way.
What chalking tells you about timing
Chalking is one of the clearest signs that an exterior is due. Caught at the light or moderate stage, the repaint is mostly washing, minor repairs and paint. Left until the coating has eroded through to bare stucco or wood, the job grows. Other signs it is time to repaint.
Let someone test it with you
Checking the surface comes right after the wash in Ebenezer’s exterior process, and chalk is part of that check. At the estimate, ask him to run the test with you on a sunny wall and a shaded one, so you see the result rather than hear it described. It tells you both how much washing and priming your house needs before paint. More on exterior painting, or on older stucco homes in Chula Vista and Spring Valley.
If your hand comes away colored, ask for a free estimate and he will tell you what it means for your walls.
Common questions
Is chalking a sign that my paint was poor quality?
Not necessarily. All exterior paint chalks eventually under San Diego sun, because ultraviolet light breaks down the binder over time. Paints with less binder relative to pigment chalk sooner and more heavily, and paint applied too thin chalks through sooner. Heavy chalking within a few years of a repaint is worth asking about.
Can I just paint over a chalky wall?
No. New paint will stick to the chalk, and the chalk is barely attached to the wall. The result is a coating that can peel or flake well before its time, sometimes in sheets. The wall has to be washed until the chalk test comes back nearly clean, and any small remaining chalk is handled with a suitable primer.
What are the white streaks on the concrete below my walls?
Often that is chalk washing down. Rain and sprinklers rinse loose pigment off a chalking wall and leave it on the walkway, the foundation or brick below. It is a clear sign the wall is chalking heavily, and those surfaces usually need cleaning along with the wall.
Does chalking happen on wood and metal too?
Yes. Any painted exterior surface in strong sun can chalk, including wood trim, siding, metal doors and old aluminum siding, which can chalk very heavily. The test is the same on every surface, and so is the rule: clean it off before recoating.