How can I tell if the dark spots on my wall are mildew or dirt?

Put a drop of diluted household bleach on a spot, away from plants and with gloves on. If it lightens within a minute or two, it is almost certainly mildew. If it stays the same, it is more likely dirt, soot or a stain. Green or slippery film low on a wall is usually algae, which also responds to treatment.

Will painting over mildew kill it?

No. Paint seals the growth in, it does not kill it, and mildew can keep living on the moisture and organic matter trapped under the film. It usually shows through again as gray or black spotting, sometimes within months. Growth has to be treated, rinsed and the wall allowed to dry fully before anything is coated.

Does exterior paint contain anything to resist mildew?

Many quality exterior acrylics do. Sherwin-Williams Duration, for example, contains agents that inhibit mold and mildew on the surface of the paint film; the label of any product will say whether it does. They work on a clean, sound surface and help keep it that way. They do not fix a wall that is constantly wet, and they cannot reach growth that was painted over rather than removed.

Why does mildew keep coming back on the same wall?

Because the conditions that grew it are still there: shade, poor airflow, a sprinkler hitting the wall, a plant pressed against the stucco or soil piled above the base. A repaint without changing those conditions only buys time. Trimming back growth and adjusting the irrigation often does more for that wall than any coating.

Mildew and Algae on Shaded Walls

The dark spots and green film on shaded, north-facing walls are usually mildew or algae, fed by morning damp, sprinkler water and shade that never lets the wall dry. Washing alone moves it around; it has to be treated and killed, rinsed, and the wall left to dry before priming and painting, or it grows back through the new coat.

Walk around almost any house in San Diego County and look at the north side. On many homes, that wall looks different from the rest: duller, darker toward the bottom, with gray speckles or a faint green tint where the sun never quite reaches. Homeowners often take it for dirt. Usually it is something alive.

What is growing on my wall?

Mostly mildew, a fungus that shows as gray, brown or black spotting, and algae, which shows as a green or olive film, often near the ground or beneath eaves. Both need moisture, shade and something to feed on, such as dust, pollen or plant debris settled in the stucco texture. They grow on the surface of the paint, and in porous stucco they work into the texture itself.

Why it grows where it does

The pattern on a house is rarely random. Growth follows four conditions, and most affected walls have at least two of them.

Shade. North-facing walls in San Diego get little or no direct sun for much of the year. Add a neighbor’s house close by on a narrow lot, a deep eave, or a mature tree, and some walls almost never dry out completely after a damp morning.

Morning moisture. The marine layer and dew wet exterior walls through much of late spring and early summer, and at the coast for much of the year. On a sunny wall that moisture burns off by mid-morning. On a shaded one it can sit until the afternoon. In Imperial Beach, salt air adds to it, because salt holds moisture against a surface long after the air has dried.

Irrigation. A sprinkler head that wets the bottom few feet of a wall every morning is an ideal place for algae. So is a drip line under a planter set against the house.

Poor airflow and debris. Shrubs pressed against stucco, vines on the wall and tree litter all hold damp and feed growth. In Bonita, where large lots and mature trees are common, the shaded strip under the canopy is often where a repaint needs the most attention. On sloped lots in Spring Valley, the wall tucked into the hillside is the one that stays damp.

Why a power wash alone does not solve it

A strong wash takes off the visible layer, and for a few weeks the wall looks clean. But spores and roots stay in the texture of the stucco and in the pores of old paint. Blasting an untreated wall can also drive growth deeper into the surface or spread it across clean areas. Within one damp season it is back, now under the new coat if the wall was painted.

The growth has to be killed before it is washed off.

How growth is dealt with before painting

Identify it. Confirm it is growth rather than dirt or staining, wall by wall.
Protect plants and surfaces around the treatment area, because cleaning solutions are not kind to landscaping.
Apply a mildew treatment suited to the surface and let it dwell for the time its label calls for, so it reaches into the texture.
Scrub where needed, particularly on heavy algae near the ground.
Rinse thoroughly with clean water so no residue remains to interfere with the paint.
Let the wall dry fully. Shaded walls take longer, sometimes into the next day, and painting a damp wall traps the moisture that the growth needs.
Prime and paint with a quality exterior coating, which often contains mildewcides that help keep the clean surface clean.

This all sits inside the first two steps of the standard exterior process, the wash and the repair check, before masking and paint.

Changing the conditions

The coating is only half the answer. The other half is the wall’s environment, and some of it is in your hands:

Redirect sprinklers so they water the garden and not the house.
Trim shrubs and branches back from the wall so air can move and light can reach it.
Keep soil and mulch below the metal weep screed at the base of the stucco.
Clean gutters so overflow does not soak the shaded walls in the winter rains.
Rinse growth-prone walls gently once a year, before it gets established again.

At the estimate, ask Ebenezer which of these apply to your house so you can arrange tree work or irrigation changes before the job rather than discover them on the day.

What growth tells you about the rest of the wall

A wall that grows mildew is a wall that stays wet, and constant damp has other effects worth checking while the house is being prepared. On wood, look for soft fascia ends, window sills and the bottom edges of trim. On stucco, look for staining or efflorescence, the white crystalline bloom left by water moving through masonry. Those are signs that the wall’s moisture problem goes deeper than its surface, and they are part of what the repair check is for.

Will a different paint fix it?

Sheen and quality help. A smoother, slightly higher-sheen finish holds less dirt for growth to feed on than a very flat one, and a quality paint resists growth better than a low-grade one. But no coating will stay clean on a wall that never dries. The lasting fix is a properly cleaned wall, the right coating, and a change in whatever keeps that wall wet.

If one side of your house looks permanently grubby no matter how often it is washed, book a free estimate and Ebenezer will tell you what is growing, why, and what it would take to keep it from coming back. More about exterior painting, and about the shaded, leafy conditions in Bonita.