White Powder on Stucco: Efflorescence, and How to Paint After It

If your stucco has a white, chalky crust, white streaks running down from a ledge, or a fuzzy white bloom near the ground, it is usually efflorescence. It is not paint failing on its own, and it is not dirt. It is minerals from inside the wall, carried to the surface by water and left behind when the water dries.

That is why it matters before a repaint. The deposit is a symptom. The cause is water moving through the wall.

What it is

The trade magazine Walls and Ceilings defines efflorescence as “the surface presence of soluble salts contained within cement, lime, sand and water.” Stucco and concrete block are full of those ingredients. When water soaks into the wall, it dissolves some of the salts. When it comes back out and evaporates, the salts stay on the face of the wall.

So efflorescence needs three things: salts in the material, water to carry them, and a path for that water to reach the surface. Take away the water and it stops.

How to tell it apart from other white marks

What you see	Likely cause	Quick check
Crusty or fuzzy white deposit, often near the ground, under windows or below ledges	Efflorescence	Dissolves or softens with water; comes back after rain
Fine white powder that rubs onto your hand evenly across a sunny wall	Chalking paint	Wipe a dark cloth across the wall; see our chalking paint page
Green, gray or black growth, worst on shaded walls	Mildew	See our mildew on stucco page

Efflorescence and mildew often turn up on the same wet walls, because both need moisture. They are cleaned and treated differently, so it is worth knowing which one you have.

Where the water comes from on a Florida house

• Sprinklers hitting the same wall every morning, keeping the bottom of the stucco wet.
• Soil, mulch or pavers piled against the base of the wall, so the stucco wicks water up.
• Gaps at windows, doors and flashing that let rain into the wall. A University of Florida study found stucco itself was wrongly blamed for some leaky homes: in simulated rain, water did not soak through stucco-coated panels even after 10 hours. The leaks came from somewhere else.
• New stucco or a fresh repair. Fresh cement-based material is wet and strongly alkaline while it cures, and the early bloom on new work is common.

Sherwin-Williams ties the last two together: fresh materials run a pH of 13 to 14, and moisture getting back into the wall “may reactivate the high pH alkalinity, resulting in efflorescence, blistering, and peeling of the paint film.”

Fix it in this order

1. Find and stop the water. Redirect sprinklers, pull soil and mulch back from the base of the wall, and seal the gaps at windows, doors and flashing. If the source is a leak inside the wall, that is a repair before it is a paint job.
2. Let the wall dry. In a Sarasota summer that takes longer than you think, especially on shaded walls.
3. Remove the deposit. Start with a stiff dry brush, then water. Walls and Ceilings warns against aggressive scrubbing that can wear off the finish or discolor it, and says to never apply an acid-based product to a dry wall. The deposit “must be removed before applying another coat of anything.”
4. Prime with a masonry primer. On stucco, a conditioner or masonry primer goes on before the finish coats. Sherwin-Williams markets its Loxon Conditioner for masonry with “outstanding alkali and efflorescence resistance” (a manufacturer claim).
5. Then paint. Two finish coats over a sound, dry, primed wall.

If the white comes back after all that, the water was not fixed. More paint will not change it.

New stucco: wait before you paint

If the efflorescence is on new stucco or a large repair, the wall may simply be too young for paint. Sherwin-Williams says new surfaces should cure “usually about 30 days.” If painting cannot wait, its guidance is at least 7 days of cure and a pH below 13, then a primer made for high alkalinity, Loxon Acrylic Masonry Primer.

On a Sarasota repaint

Efflorescence is one of the things a good estimate should point out before quoting, because painting over it fails. Every exterior our family-run Sarasota crew paints starts with a wash, then scraping, mildew treatment, caulking and priming before any color goes on, using Sherwin-Williams coatings. An active water problem should be named before anyone prices the paint, because no coating fixes it.

For cracks in the same wall, see stucco cracks before painting. For the full process, see exterior painting.