Mildew on exterior paint and siding is a fungus that grows as gray, black or green spots, usually on shaded, damp walls. It looks like dirt, but a drop of household bleach will lighten mildew within a minute or two and leave dirt unchanged. The fix is to kill and wash it off, let the surface dry, and repaint promptly with a paint containing a mildewcide, while adding sun and airflow so it does not return. This page follows Purdue Extension and USDA Forest Products Laboratory guidance and is general information.
On this page: Signs · Causes · Bleach test · Fixes · Basis · When to call
Signs of mildew
- Small gray, black or green spots, often in clusters, that spread over time.
- Growth heaviest on north walls, under eaves, behind shrubs and near downspouts.
- Spots that come back within a season after washing.
- Spotting on caulk lines and on the shaded underside of trim and soffits.
Rust-colored streaks at nail heads and brown streaks on cedar or redwood are different problems (corroding nails and wood extractives), and they do not bleach out.
Where and why mildew grows
- Shade and moisture. Mildew is most common in shaded or protected areas that stay damp.
- Plants and sprinklers. Shrubs against the wall block sun and air; sprinklers that hit the siding keep it wet.
- Gutter and downspout leaks soak the same patch of wall every storm.
- Paint type. Flat finishes and paints containing linseed oil are more susceptible; zinc oxide pigment inhibits growth.
- Painting over live mildew. Spores under new paint grow right back through it.
Even in a dry climate, a north wall behind a juniper hedge can stay damp enough to grow mildew.
The one-minute bleach test
- Pick a small dark spot at eye level.
- Apply a drop or two of fresh household bleach (about 5 percent sodium hypochlorite).
- Wait one to two minutes. If the spot lightens, it is mildew. If not, it is probably dirt.
- Rinse the spot with water.
The correct fix, and the wrong ones
| Fix | When it fits |
|---|---|
| Scrub with 1/3 cup detergent, 1 quart household bleach and 3 quarts warm water; rinse well; let dry | Mildew confirmed by the bleach test. |
| Repaint soon after cleaning with a topcoat containing zinc oxide and a mildewcide, over a primer | Repainting a wall that has had mildew. |
| Trim plants back, redirect sprinklers and fix gutter leaks | Mildew returns to the same area. |
| Use enamel or satin finishes rather than flat on problem walls | Shaded, damp elevations. |
Wrong fixes: painting over mildew, mixing bleach with ammonia or ammonia-based cleaners (toxic fumes), or high-pressure washing that forces water behind siding and into the wall.
Research basis
- Purdue Extension and USDA FPL, NCR-134 Discoloration of House Paint. Describes the bleach test, the detergent and bleach cleaning solution and the warning against mixing bleach with ammonia; notes mildew is more common in shade, on flat paint and on linseed-oil paints; zinc oxide inhibits mildew and titanium dioxide has little effect; recommends a zinc oxide and mildewcide topcoat over a primer.
- NCR-133 Paint Failure Problems. Paint should not be applied over a surface that has not been cleaned, which applies to mildewed walls as much as chalky ones.
For other paint failures, see peeling and blistering paint and chalking and fading. If damp walls trace back to leaking gutters, see gutters pulling away.
When to call
Call when mildew covers large areas, returns right after cleaning, or sits over soft or swollen siding, which can mean water is getting into the wall. Integrity cleans, primes and paints exteriors and fixes the gutter and flashing problems that keep walls wet. See our Colorado Springs painting page and the exterior problems guide.
Book a paint assessment
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. Colorado Springs area: call (719) 488-2800 or book a free inspection. Denver metro: call (303) 209-0152 or book a free Denver inspection. Austin: call (512) 610-5897 or book a free Austin inspection. We reply the same business day.
Sources: Cassens and Feist, NCR-134 Discoloration of House Paint (Purdue Extension and USDA FPL); Cassens and Feist, NCR-133 Paint Failure Problems and Their Cure.
