The black spots on your paint are mildew, and painting over them will not work

Black staining on a north wall is almost always mildew, not dirt. The bleach test proves which one you have, and why painting over it fails within a season.

Those gray-black patches on the north side of the house are almost always mildew, not dirt, and the way to know for certain is a five-minute test with diluted household bleach. Dab a small amount on the stained area and wait a few minutes. If the spot lightens or disappears, it was mildew, a living organism that bleach kills. If nothing changes, you are looking at dirt, pollen film, or a stain from something else, and it will come off with soap and water instead. That distinction decides everything about how the wall gets fixed, because dirt and mildew are treated completely differently, and guessing wrong is the single most common reason a repaint fails early.

Why it always finds the same wall

Mildew is a fungus, and it needs three things to establish itself on a painted surface: moisture, organic material to feed on, and shade from direct sun. A painted wall already supplies the second one, since dust, pollen, and the paint film itself carry enough organic matter to feed it. The other two are why the growth shows up in the same places on nearly every house: the north elevation, which gets the least direct sun of any wall and dries slowest after rain; anywhere under an eave or soffit overhang, which blocks both sun and rain but not humidity; behind shrubs planted close to the siding, which cuts airflow and holds moisture against the wall; and any spot that catches irrigation spray, which resets the clock on drying every time the sprinklers run. Put simply, mildew grows wherever a wall stays damp past mid-morning. In Northeast Florida, with humidity that sits high for most of the year and a warm season that runs long, those conditions exist on some part of almost every house, almost every month.

Why painting over it is the mistake that costs a repaint

Painting over mildew without killing it first is the single most common error homeowners and even some crews make, and it never works. Mildew growing on a wall does not sit only on the surface, it roots into the microscopic texture of the old paint film. A fresh coat applied on top seals the organism in rather than removing it, and it keeps growing underneath, drawing moisture and nutrients from the new film exactly as it did the old one. The black pattern telegraphs back through within a season, sometimes faster on a north wall that never gets the sun exposure to dry out and slow the growth. This is why the prep on a mildew-affected elevation is the actual job. The paint itself is almost never what determines whether the problem comes back.

What actually kills it, and why rinsing matters as much as applying

Killing mildew, rather than just knocking the visible growth off, takes a cleaning solution with real dwell time. A diluted bleach solution or a dedicated mildewcide cleaner needs to sit on the surface for a period, generally in the range of ten to fifteen minutes, before it has actually killed the organism rather than just bleaching the color out of it. Skip the dwell time and the wall can look clean for a week or two before the surviving growth reasserts itself. The second half of the job matters just as much: a thorough rinse afterward. Cleaning residue left on the wall interferes with paint adhesion, so a wash that kills the mildew but isn’t rinsed well enough sets up a different failure a few months later, one that shows up as peeling rather than staining.

Pressure washing is the wrong tool for cleaning a painted wall

Pressure washing has a place here, at low pressure, mainly for rinsing after the cleaning solution has done its work. It is not the right way to clean a painted exterior. High pressure drives water into seams, behind lap siding, and into wood at joints and end grain, and that moisture does not show up as damage right away. It shows up weeks or months later as blistering, softened trim, or paint lifting from underneath. The right approach on a painted house is a soft wash: low pressure, with the cleaning chemistry doing the work of killing the mildew rather than the water pressure trying to blast it off. A crew washing a house ahead of a paint job should be doing exactly this as part of prep, not running a pressure washer at full force and calling it clean.

A mildewcide paint slows recurrence, it does not prevent it

Most quality exterior paints today include a mildewcide additive, and it genuinely helps: it makes the cured film more resistant to new mildew taking hold on the surface. What it does not do is substitute for killing what is already growing on the wall before that paint goes on, and it does not change the underlying conditions, the shade and the moisture, that made the wall a good host for mildew in the first place. Think of it as raising the wall’s resistance, not removing the cause. A north elevation with a mildewcide-treated paint and a sprinkler head hitting it twice a day will still grow mildew again. It will likely just take longer to show.

Sheen is a real factor, not a marketing detail

The finish sheen affects how much moisture and dirt a painted surface holds onto. A flat or matte film is more porous, and that texture gives both water and organic material more to cling to, which is exactly what mildew needs. A satin or other low-luster sheen sheds water faster and holds less surface grime, which means less for mildew to feed on and an easier surface to wash if it does show up. On an elevation that already fights shade and damp, sheen is worth choosing deliberately rather than defaulting to flat because it looks a certain way on the sample card. Paint color and how it holds up in this climate covers the fuller tradeoff between sheen, color, and durability.

The cause is usually water, not the wall

A wall that keeps growing mildew after a proper cleaning and repaint almost always has something feeding it moisture beyond ordinary weather. The usual culprits are irrigation heads aimed at or too close to the siding, shrubs and hedges planted tight against the house that block airflow and trap humidity against the wall, gutters overflowing onto the siding below because they’re clogged or undersized, downspouts that dump water right at the foundation instead of carrying it away, and grading that slopes toward the house instead of away from it, keeping the base of the wall damp long after rain elsewhere has dried. None of those are a painting fix. A repaint, even a properly prepped one, will not outlast a sprinkler head that keeps soaking the same six feet of wall twice a day. If one of these is the actual source, it’s worth getting the right trade in, an irrigation tech to redirect a head, a landscaper to pull shrubs back off the siding, or a gutter service to clear and reset the downspouts, before or alongside the repaint. Fixing the water source is what makes the paint job last.

When it’s more than a coating problem

Mildew that responds to a bleach test, stays confined to the painted surface, and returns only on the same shaded elevation is a surface and exposure problem, and it’s treatable with the cleaning and prep described above. A different set of signs points somewhere more serious: siding that feels soft or spongy when pressed, paint bubbling or peeling in patches that have nothing to do with sun exposure, a musty smell inside the house along that same wall, staining that comes back within days of a proper cleaning even in a stretch of dry weather, or discoloration on the interior drywall backing that same exterior wall. Those point to moisture getting into the wall assembly itself, from a flashing failure, a chronic leak, or a water source that never fully dries out, and no amount of exterior paint addresses that. If you see any of those signs, the right move is a qualified assessor who can find where the water is actually getting in, not another repaint.

Mildew in a bathroom or around a window is caused by a different mechanism than the mildew on a north wall outside. Indoors, it’s driven by condensation and inadequate ventilation rather than by shade and sprinkler spray, and it needs a different fix: better exhaust fan use, addressing condensation at the window itself, and a paint suited to a consistently humid room. It’s worth treating as its own issue rather than assuming whatever caused the exterior staining is the same cause indoors. Interior painting work on a bathroom or a chronically humid room should account for that difference in prep and product from the start.

What a mildew-affected elevation should look like on an estimate

Two estimates are only comparable if both name the same things for the affected wall. A proper quote for a mildew elevation should say what cleaning method will be used and at what pressure, whether the solution is being left to dwell long enough to actually kill the growth rather than just wash it, how the wall will be rinsed, how much dry time is planned before priming, whether the paint specified includes a mildewcide additive, and what sheen is going on that elevation. A quote that just says “pressure wash and paint” doesn’t tell you whether the mildew problem was actually addressed or just covered up again.

If the north side of your house has gone dark, run the bleach test before you do anything else, and if it comes back positive, look for what’s keeping that wall damp before booking the repaint. Exterior painting across Jacksonville, Ponte Vedra Beach, Nocatee, St. Augustine, Fernandina Beach, Yulee, Middleburg, and Palm Coast should start with that same diagnosis on any elevation that’s staining, whether that’s a first repaint or a return visit. For more on getting a Jacksonville exterior ready before the crew arrives, see what should be inspected before repainting and how often a house exterior needs repainting in Northeast Florida, which covers mildew as one part of a broader look at what shortens a paint job’s life. Homeowners in Jacksonville dealing with a recurring north-wall stain can start with the bleach test and go from there.