How to remove mildew from painted walls
Small black or gray spots on a painted wall are often mildew, a surface mold that grows where the wall stays damp. In fall and winter it tends to show up on outside walls, behind dressers and beds, in closets and in laundry rooms. Warm, moist room air hits the cold wall and leaves a film of water the spores can live on. A small patch is usually a job you can do yourself. If you don't fix the damp, it will likely come back.
Why mildew grows on painted walls
Mold spores float in nearly all indoor and outdoor air. They can't grow without moisture, and that's the part you can control. In cold weather, an outside wall can be several degrees cooler than the room. Warm, damp air touching it leaves water on the paint, like on a cold glass. Behind a dresser, bed or stack of boxes, the air barely moves, so that film stays wet and mildew grows.
Mildew is the everyday name for flat surface mold. It often looks like tiny black, gray or green dots and smells musty. If you clean it but leave the damp, it usually comes back in the same spot. If it keeps showing up through fresh paint, or the drywall feels soft or looks stained from behind, the moisture may be inside the wall. That can mean a leak or a gap in insulation, which needs a fix at the source.
Why mildew keeps showing up
Still air behind it lets condensation sit on the paint.
Drying laundry, cooking, showers and people add moisture to the air.
Closets, laundry rooms and shut rooms trap damp air.
Plumbing, a window or outside water keeps one area wet.
Missing insulation leaves a patch cooler than the rest.
Soot and grime can look similar but don't grow.
| You notice | Likely cause | Start at |
|---|---|---|
| Not sure it's mildew, or how big | Spot test and measure | Step 1 |
| Spots return on an outside wall | Find and fix the moisture | Step 2 |
| Ready to clean a small patch | Gloves, N95, goggles, air flow | Step 3 |
| Light spots on sound paint | Detergent and water wash | Step 4 |
| Dark stains after washing | Bleach solution per label, rinse, dry | Step 5 |
| Stained paint or soft drywall | Prime and repaint, or replace | Step 6 |
Let's fix it
- Check that it's mildew, and how big it isDirt and soot can look a lot like mildew, so test first. Put on gloves. Mix a little household bleach with water, about 1 part bleach to 10 parts water in a small cup. Dab one spot with a cotton swab and wait a minute or two. Mildew often lightens or fades quickly. Dirt usually stays the same. Next, measure the patch. If the total area is over about 10 square feet, stop and call a pro, as the EPA advises. Do the same after flooding or sewage, or if someone in the home has health concerns. Spots low on an outside wall or behind furniture point to condensation.
- Find and fix the moisture firstCleaning won't last if the wall stays damp, so do this before you wash. Pull beds, dressers and boxes at least a few inches off outside walls so air can move behind them. Check the room's humidity with a hygrometer, a small gauge that costs about $10 to $20. The EPA suggests keeping it below 60 percent, ideally 30 to 50 percent. Run bath and kitchen fans, and use a dehumidifier in damp rooms. Then feel for a wet patch and look outside for overflowing gutters at that spot. If you find a leak inside the wall, stop and get it repaired first.
- Gear up and get air movingMildew can release spores when you scrub, and cleaners give off fumes. Put on long rubber or nitrile gloves that reach your forearms. Wear an N95 respirator, which hardware stores sell, and goggles without vent holes. Open a window and set a box fan in it blowing out, so air flows away from you. If the spots are near a switch or receptacle, switch off that circuit's breaker before you wash around it, and keep the sponge damp, not dripping.
- Wash with detergent and waterThe EPA advises scrubbing mold off hard surfaces with detergent and water. Mix a squirt of dish soap or laundry detergent in a bucket of warm water. A store mildew cleaner made for walls also works; follow its label and test it in a hidden spot first. Dip a soft sponge or soft brush and wring it out well. Scrub the spots gently in small circles, working from the clean edge toward the middle so you don't spread them. Rinse the sponge often in a second bucket of plain water. Wipe the area with a clean damp cloth to rinse. Pat it dry with towels.
- Use bleach only if stains remain, then dryIf dark stains are left after washing, a bleach solution can lighten them on paint that passed the test in Step 1. Use a regular household bleach and follow the dilution on its label for hard surfaces. Never mix bleach with ammonia, vinegar or other cleaners, since toxic fumes can form. Apply it with a sponge and leave it on only as long as the label says. Rinse if the label says to. Dry the wall fully. Run a fan or dehumidifier for a day or more, and aim to dry it within a day or two, as the EPA advises.
- Prime and repaint, or replace soft drywallOnly paint once the wall is clean and fully dry. The EPA warns against painting or caulking over moldy surfaces. If a shadow of the stain remains, roll on a stain-blocking primer and let it dry as the can directs. Then apply two coats of an interior paint labeled mildew-resistant. If the house was built before 1978, test the old paint for lead before you sand or scrape. Press the wall with your thumb. If the drywall feels soft, crumbles or is stained through from behind, cleaning won't save it. Cut out the damaged section and patch it, or call a pro for anything bigger than a small patch.
Watch it done
- The spots are gone and the wall looks even with a light shining across it
- The wall is dry to the touch, with no musty smell
- Room humidity stays between about 30 and 50 percent
- No new spots show up after a few weeks of cold weather
If it comes back
If the mildew is in a bathroom, on the ceiling or around the tub, the cause and fix are a bit different. See the guide to mold on a bathroom ceiling, and check that the fan really pulls air with the guide to a noisy or weak bathroom fan.
If your windows drip every cold morning, the air in the house is likely too damp. See the guide to condensation on windows. If your dehumidifier runs but the bucket stays empty, see the guide to a dehumidifier that isn't collecting water.
If the drywall was soft and you cut it out, see the guide to patching a hole in drywall. If the paint peels or bubbles after cleaning, see the guide to fixing peeling paint, and the guide to touching up wall paint to blend in a small repair.
What a pro typically charges (a rough estimate; prices vary by area): DIY supplies usually cost about $10–$60, including primer and paint. Mold remediation pros often charge a few hundred dollars for a small area, and much more when walls must be opened or a leak repaired.
Find a mold remediation pro near you