Mildew and Mold on Coatings
Mildew and mold are fungal growths that discolor painted surfaces in damp, shaded or poorly ventilated areas. Removal, moisture control and suitable coatings prevent recurrence.
Mildew and mold are common names for fungi that grow on surfaces, including paint and coatings, when moisture, nutrients and suitable temperatures are present. On coatings, they appear as black, gray, green or brown spots and streaks. Mildew is a cosmetic and hygiene problem rather than a failure of the film itself, but persistent growth can stain coatings permanently, hold moisture against the surface and contribute to degradation. It is one of the most common complaints on exterior house paint, roofs, bathrooms, food facilities and cold rooms.
Appearance
- Dark spots, often in clusters, that spread over time.
- Growth heavier on shaded, north-facing or sheltered surfaces.
- Streaks under eaves, gutters and window sills where water runs or lingers.
- Musty odor indoors.
- Growth may be mistaken for dirt, soot or bleeding and staining.
Simple field test
A common field check is to apply a few drops of diluted household bleach to the stain. If the spot lightens within a couple of minutes, it is probably mildew. If not, it is likely dirt or another stain. Wear gloves and eye protection.
Why fungi grow on coatings
Fungi need:
- Moisture. High humidity, condensation, poor drainage or frequent wetting.
- Nutrients. Dirt, pollen, organic residues, and some coating components such as certain oils and thickeners.
- Suitable temperature. Most common molds grow well at ordinary building temperatures.
- Limited sunlight. UV inhibits many species, so growth concentrates in shade.
Conditions that encourage growth include:
- Shaded walls with dense vegetation.
- Poor ventilation in bathrooms, kitchens and basements.
- Cold surfaces where condensation forms (see dehumidification and climate control).
- Coatings that hold dirt, such as soft or tacky films.
- Paint applied over existing, untreated mildew.
Coating factors
- Binder type. Some oil and alkyd paints can provide food for fungi more readily than many acrylics.
- Sheen. Flat paints have more surface texture that holds dirt and moisture; higher-sheen finishes clean more easily (see architectural paints).
- Biocides. Many paints include mildewcides (in-film fungicides) to slow growth. These deplete over time through weathering and leaching.
- Dirt pickup. Soft or tacky films accumulate dirt that feeds fungi.
Health considerations
Some molds produce allergens and irritants. Large indoor mold problems may need professional assessment and remediation following applicable guidance. Wear appropriate respiratory protection and gloves when cleaning heavy growth (see respiratory protection).
Removal
- Identify and fix moisture sources: leaks, condensation, poor drainage, missing ventilation.
- Protect plants and nearby surfaces before using cleaning solutions.
- Clean the surface with a mildew cleaning solution. Common approaches use diluted sodium hypochlorite (bleach) with detergent, or proprietary mildew removers. Never mix bleach with ammonia or acids.
- Allow dwell time as the product directs, then scrub with a soft brush.
- Rinse thoroughly with clean water.
- Allow to dry fully before painting.
Pressure washing can help but may drive water into wood and masonry. Use low pressure on soft substrates (see water jetting for industrial methods).
Recoating after mildew
- Never paint over live mildew. It can grow through the new coat.
- Remove any coating that has lost adhesion.
- Use a primer compatible with the substrate and topcoat.
- Choose a topcoat with in-film mildewcide for susceptible locations.
- Consider higher-sheen finishes in damp interiors.
Prevention
- Control moisture. Vent bathrooms, kitchens and laundry rooms. Use dehumidifiers. Maintain gutters and drainage.
- Increase sunlight and airflow. Trim vegetation away from walls.
- Clean regularly before growth becomes established.
- Select suitable coatings for the environment, such as products formulated for high-humidity rooms or food and beverage facility coatings for processing plants.
- Insulate cold surfaces to reduce condensation.
Industrial and roof environments
Mold, algae and lichen also grow on roof coatings, cooling towers and tanks. On light-colored roof coatings, growth reduces reflectivity and can make the surface look aged (see roof coatings). Regular cleaning with methods approved by the coating manufacturer, and coatings designed to resist dirt pickup, help maintain appearance.
Mildew versus algae
Algae often appear green and grow on wet, sunlit or partly shaded surfaces, including masonry and roofs. Mildew is usually black or gray. Both are biological growths and are treated in similar ways, though product choices may differ.
In practice
In practice, cleaning comes before any recoat. Growth left under a new coating can continue to feed and stain through the film. A typical approach is to wash with a suitable cleaner or diluted bleach solution, rinse thoroughly, let the surface dry and then recoat with a product formulated for damp or shaded exposure. Improving ventilation, drainage and sunlight exposure, or trimming plants, often matters as much as the coating itself.
Key terms
- Mildew. A common name for fungal growth on surfaces, often black, gray or green spots.
- Mold. Fungal growth that can colonize damp surfaces and organic material.
- Algae. Plant-like organisms that grow on damp, sunlit surfaces, often green.
- Biocide. An ingredient that discourages growth in the film (in-can preservatives protect the wet product).
- Bleach test. A drop of household bleach on a stain; mildew usually lightens, dirt usually does not.
Frequently asked questions
Can mildew damage paint?
Mildew mainly causes discoloration. Over time, it can trap moisture and contribute to loss of adhesion on susceptible substrates.
Do mildew-resistant paints stop growth permanently?
No. In-film biocides slow growth but deplete over time. Moisture control is the long-term solution.
Is bleach the only cleaner?
No. Several proprietary cleaners are available. Follow the label and test on a small area first.
Related entries
- Architectural paints
- Coating additives
- Microbiologically influenced corrosion
- Food and beverage facility coatings
- Humidity and condensation testing (new)
- Animal facility floor coatings (new)
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
