Painting guide
Mildew on Nashville Siding: Clean, Treat, or Repaint?
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Dark patches have appeared on the siding, usually low on the wall or on an elevation that gets little sun. They were not there last year, or they were and you told yourself it was dirt.
There are three possible answers, and picking one starts with establishing what you are looking at, because dirt and mildew look similar at a distance and want different work.
The word most people reach for is mold, and what matters is where it is. Growth on the outside face of siding is a cleaning and coating question; growth inside damp building materials is a different subject and nothing here addresses it. So make one check first: if the growth outside sits opposite a wall that is damp, stained or musty inside, the wall is holding water and that water gets found before anything is done to the paint.
Is it growth or is it dirt?
Growth and ordinary dirt look similar from ten feet away and want completely different work, so the first job is telling them apart.
The common test uses household bleach diluted in water, applied to a small patch, left briefly and rinsed off. Take the dilution from the label on the bottle you are holding, because household bleach is not sold at a single concentration. Biological growth lightens or disappears under it; dirt does not change.
Three cautions come first and none is optional.
Never mix bleach with any other cleaner. The CDC’s position is that mixing bleach solutions with vinegar or ammonia, or applying heat to them, can generate chlorine and chloramine gases, and that inhaling those gases can cause severe lung tissue damage. This happens to people who were being helpful by using two products instead of one, and that includes reaching for a second product because the first one seemed slow.
Protect your eyes and skin, and think about what is below. Solution running down a wall reaches foundation plantings. Wet them down first so the runoff arrives diluted, or cover them.
Test somewhere inconspicuous, and know what the wall is made of. Bleach affects some finishes and materials, painted metal and anodized or mill-finish aluminum among them. Those want a hidden test patch and a proper rinse rather than a confident start on the front elevation.
If you are not comfortable handling bleach, proprietary exterior cleaners formulated for this exist that do not use it, and the test can wait. Getting the identification right matters more than getting it today.
If the patch lightens, you have biological growth. If it does not change, you have dirt or staining, and that is a washing question rather than a treatment one.
If it is growth, there is one more thing worth understanding before deciding what to do about it, which is why it chose that particular wall.
Why it is on that wall and not the others
Mildew needs moisture, and it grows where surfaces stay damp longest. That is a small number of predictable places.
North elevations and anything permanently shaded. A wall that never gets direct sun dries slowly after every rain and holds dew well into the morning.
Under deep overhangs. Sheltered from the rain, but equally sheltered from the sun and moving air, so dew settles and stays and rainfall never washes the surface.
Low on the wall, near grade. Splash from the ground keeps the bottom courses wet, and the ground itself holds moisture against the wall.
Behind plantings. Shrubs against a wall block both sun and airflow. A wall behind a hedge is a wall that stays damp.
Anywhere water discharges. Look under a downspout, beside an outdoor faucet, and below any air conditioning condensate line, because each of those delivers water to the same square foot of wall over and over.
None of the three answers below changes why the wall was damp. Work out the extent first: if the fence line, the deck boards and a shed’s shaded side carry the same growth, the conditions are site-wide and any treatment holds for a shorter time than on a house with one damp corner. Then, in rough order of effect: airflow, since cutting plantings back lets air move across the wall between wettings; sun, since thinning a branch that shades an elevation changes how long it stays wet; and water delivery, usually the quickest, meaning a downspout, an overflowing gutter or an irrigation head reaching the siding. Painting more often, choosing a darker color and cleaning on a shorter cycle change none of it.
Clean it
Cleaning is the answer where the growth is light, the coating underneath is still sound, and the conditions that produced it can be improved. All three have to hold together.
Sound has a specific meaning here, and it gets checked rather than assumed. One of the checks below lifts paint, so on a building that predates 1978 the lead-safe question is answered before a blade goes under any edge: Painting a Pre-1978 Nashville Home: When EPA Lead-Safe Rules May Apply. No powder should transfer to your hand when you rub the wall. Nothing should lift when you run a putty knife under an edge. There should be no cracking, and the film should still hold some sheen in the areas the growth has not reached. If the wall fails any of those, cleaning is still necessary but it is no longer the answer.
Clean the growth off, rinse well, let the wall dry, and then look at it again in daylight. A wall that comes back to something close to its original color was dirty and is now clean, and that is the end of the job for now.
How long that lasts is not decided by how well you cleaned it.
Treat it
Treatment becomes the answer when the growth is established rather than light, the coating is still sound, and a straightforward cleaning has already been tried and did not hold.
The difference between cleaning and treating is the difference between removing growth from a surface and killing it. Washing takes off what is on the face of the wall. What sits in the texture of the coating, in the grain of an exposed board, and in every joint and lap survives the wash and starts again from there, which is why a wall can look clean in the afternoon and speckled again before the season is out.
In practice that means a product with a dwell time rather than a scrub and a rinse. Two things matter across all of them: the solution stays wet on the surface for the time the label specifies, and the surface is rinsed properly afterwards so nothing is left to interfere with a future coating. A product applied and immediately washed off has been used as a cleaner rather than as a treatment, which is how this step usually gets wasted. The label governs, not any general description including this one.
Two things follow that are easy to skip. Painting over live growth risks it continuing underneath the new film, where nothing can be done about it. And a wall that has been treated and rinsed still has every condition that produced the growth in the first place, so the work of changing those conditions is not optional here either. Treatment resets the clock. It does not change what the clock is counting.
Repaint it
Repainting becomes the answer when the coating itself has finished, and the growth turns out to have been what made you look closely at a wall already at the end of its life.
Four signs push a wall into this category. Powder on the hand means the binder has broken down at the surface. Film that lifts or peels is a bond failure with its own causes. Coating thin enough that the substrate shows through has stopped protecting the material. And growth that returns quickly after a proper treatment points at a film that has lost the surface properties that resisted it, which is the sign most often misread as a cleaning failure.
The practical test is how long a proper cleaning holds. A wall that stays clear for a long stretch has a coating doing its job, and cleaning is maintenance. Growth returning quickly on a film that is also chalking, thinning or lifting is a sign that cleaning has become the recurring job and the coating is why.
Even here, cleaning is still the first thing that happens, because nothing goes onto a wall carrying live growth. In this case it is preparation rather than the answer.
One warning about that preparation. A wall being clean and a wall being ready for paint are two conditions, and washing settles only the first. What settles the second is Is Pressure Washing Necessary Before Exterior Painting in Nashville?. Chalk and failing film survive a thorough wash and look better afterwards, which is what makes them easy to miss. Run the hand and the knife over the wall again after it dries, because the growth was hiding the film and now it is not.
What happens if you paint over it
Paint it directly and the growth is sealed under a film on a wall that stays damp. The new coating fails early there, and the failure reads as a paint problem to whoever looks next, so the actual cause never gets addressed.
There is a simpler consequence as well. Growth is dark, and the new color goes on unevenly over it in a way that becomes visible as soon as the wall dries.
The sequence matters if the answer turns out to be a repaint. Clean and treat first, let the wall dry properly, and only then decide what the coating needs. The reason for that order is not caution: a wall carrying a season of growth looks finished, and a wall that looks finished can turn out to be sound paint under something that washes off.
Neither of those is a reason for panic. They are reasons to spend an extra afternoon on the wall now and not an extra weekend on it once the new coating has failed in the same places.
Source. The warning about mixing bleach is the CDC’s, from Knowledge and Practices Regarding Safe Household Cleaning and Disinfection for COVID-19 Prevention, published in MMWR on 12 June 2020 and read 10 September 2026: https://www.cdc.gov/mmwr/volumes/69/wr/mm6923e2.htm. Dilution and dwell time for any product named on this page are governed by that product’s own label rather than by anything here. On the growth itself, Sherwin-Williams’ problem-solver article on mildew, opened 10 September 2026 at https://www.sherwin-williams.com/painting-contractors/products/resources/SW-ARTICLE-PRO-MILDEW, describes it as black, gray, green or brown spots, states plainly that paint does not cause mildew, and names as causes surfaces that tend to be damp or receive little or no direct sunlight, and painting over a substrate that already contains mildew.