Mildew on siding: how to identify, clean and prevent it before you paint

Kill it and clean it off before any paint goes on. Mildew is a living fungus, and paint applied over it doesn’t stop it: it keeps growing and can show through the new finish. Confirm it’s mildew with a quick bleach test, scrub it off with a mildew cleaner, rinse well, and let the siding dry completely.

Michigan’s humid summers give mildew plenty of chances, especially on walls that stay shaded and damp. Here’s how to identify it, remove it without harming your siding or your plants, and keep it from coming back, whether you’re prepping for a weekend project or a full exterior painting job.

Is it mildew or dirt?

Mildew usually shows up as gray, black or brownish specks and blotches, often in clusters, and it can look a lot like dirt. Green growth is usually algae, which gets the same treatment. Dirt tends to collect low on the wall, where rain splashes up, and near downspouts and walkways.

The quick test: put on gloves and eye protection, then dab a few drops of household bleach diluted with water onto a dark spot. If it lightens or disappears within a minute or two, it’s mildew. If it stays dark, it’s most likely dirt. Many walls have both, so test several spots.

Why does mildew grow where it does?

Mildew needs moisture, shade and something to feed on, such as the dirt, pollen and other organic film that settles on siding. Take away any one of those and it struggles.

That’s why north-facing walls are the usual trouble spots: they get little direct sun, so they stay damp longer after rain and morning dew. The same goes for siding behind shrubs and under trees, beneath porch roofs and decks, and anywhere a sprinkler, an overflowing gutter or a leaky downspout keeps the wall wet. Humid Michigan summers only help it along.

How do you clean mildew off siding safely?

Use household bleach diluted with water or a commercial mildew cleaner made for siding. A common homemade mix is about one part bleach to three parts water, sometimes with a little ammonia-free detergent, but bleach strengths vary, so follow the label of whatever you use.

1. Protect yourself. Wear eye protection, gloves and long sleeves, and keep your footing stable; a brush on an extension pole beats a ladder.
2. Protect your plants. Soak shrubs, flower beds and grass near the wall with plain water first, and cover delicate plants with breathable cloth.
3. Apply and scrub. Work one section at a time, applying the solution from the bottom up so it doesn’t streak down dry siding. Give it the time the label calls for without letting it dry, then scrub with a soft-bristle brush.
4. Rinse thoroughly. Rinse the siding from the top down with plenty of clean water, since leftover cleaner can interfere with paint adhesion. Then rinse the plants again.
5. Let it dry completely. This step decides more paint jobs than it gets credit for; more on it below.

Should you use a pressure washer on mildew?

Carefully, if at all. A pressure washer is useful for rinsing, but water pressure alone tends to blast off the visible stain without killing what’s rooted in the surface. The cleaner still has to do its work.

Too much pressure does real damage. It can gouge soft wood and raise its grain, strip paint that was still sound, and drive water up behind lap siding and into the wall, where it lingers long after the surface looks dry. Use a wide spray tip and low pressure, keep the nozzle well back, and aim downward, never up under the laps or at windows and vents.

On homes built before 1978, washing that knocks loose old paint can scatter chips that may contain lead. The EPA’s Renovation, Repair and Painting (RRP) rule requires certified firms and lead-safe work practices for paid work in pre-1978 homes that disturbs more than 6 square feet of painted surface per room inside, or more than 20 square feet outside. See the EPA’s RRP program page and our guide to lead paint in older homes.

Can you paint over mildew?

No. Paint doesn’t kill mildew that’s already on the wall. Sealed under a new coat, it can keep growing and show through as dark spots, and because the paint is stuck to a living film instead of the siding, it’s more likely to peel. A stain-blocking primer may hide the color for a while, but it isn’t meant to kill living growth.

Poor adhesion is one of the common causes covered in our guide to why exterior paint peels, and mildew is an easy one to prevent.

Many exterior paints include mildewcides that help the new film resist growth; check the label to see whether yours does. They’re a worthwhile second line of defense, especially on shaded walls. But they protect the new paint, not the surface underneath, so they’re no substitute for cleaning. We paint with Sherwin-Williams; whatever the brand, the label tells you what a paint contains and where it’s meant to go.

When is the siding ready to paint?

When it’s clean, sound and dry. Wood absorbs water during washing, and paint applied over damp wood tends to blister and peel as that moisture tries to escape. Plan on at least a day or two of dry weather, longer when it’s cool, humid or shaded, which describes exactly the walls where mildew grows. A moisture meter takes the guesswork out of it.

Before priming, re-check the trouble spots, and give any dark speck that survived cleaning another round. Then the rest of the prep follows as usual, from scraping loose paint to priming bare wood and replacing failed caulk. Our guide to good preparation walks through each step.

Timing matters, too. Michigan’s exterior season runs roughly from late spring to early fall, and slow-drying walls eat into it; our guide to when to paint an exterior in Michigan helps you plan around it.

How do you keep mildew from coming back?

You can’t make siding mildew-proof, but you can make it a much poorer home for mildew. Most of the work comes down to sun, air and water:

• Trim back plants. Keep shrubs a little away from the walls and thin overhanging branches, so the siding gets light and air.
• Clean the gutters. Overflowing gutters and leaky downspouts soak the same stretch of wall in every storm.
• Improve airflow and sun. Move firewood and anything else stacked against the house, and avoid dense plantings right at the foundation.
• Redirect sprinklers so they water the lawn, not the siding.
• Wash shaded walls each year with a mild cleaner, before mildew gets a foothold.

Our exterior paint maintenance checklist puts these into a simple yearly routine.

Questions

Planning an exterior project?See how Guiffre’s prepares and paints exteriors built for Michigan weather.→