Washing a House Before Painting It
Paint will not stick to dirt, mildew or chalk. What Sherwin-Williams and the National Park Service say about cleaning siding before a paint job, and how long to leave between the two.
The short answer
Wash first, let it dry completely, then paint. Sherwin-Williams says a surface ready for paint is clean, firm and dry, and the National Park Service recommends a detergent solution, a soft brush and a garden hose rinse as the gentlest way to clean before repainting, with any mildew killed first so it does not grow back under the new coat.
Why paint fails on a dirty wall
Paint bonds to what it touches. If that is dust, pollen, mildew or chalky old paint, the new coat bonds to that layer instead of the siding, and peels with it. The National Park Service’s guide to exterior paint problems on historic woodwork says surface deposits left on a wall before repainting can be a barrier to proper adhesion and cause peeling (NPS Preservation Brief 10).
What the paint maker says
Sherwin-Williams’ exterior prep guide treats a prepared surface as clean, firm and dry (Sherwin-Williams surface preparation):
- Wash off contamination with a suitable cleaner, rinse well, and let the surface dry.
- On vinyl siding, scrub with warm soapy water and rinse thoroughly. Do not paint vinyl a darker color than the original, which can make it warp.
- Do not paint right after rain or when rain is expected, in fog, or below 50 degrees F unless the product is made for it.
Mildew has to be killed, not painted over
Brief 10 explains that mildew is a fungus that feeds on the paint film and on dirt, thriving where it is damp and shaded: window sills, under eaves, around gutters and downspouts, north walls and behind shrubs. Its test is simple: put a drop of household bleach on the spot. Mildew turns white right away; dirt stays dirt. The brief’s mildew wash is detergent and household bleach in water, scrubbed in and rinsed, and it recommends mildew resistant primer and paint afterward.
Chalky paint
Old paint that rubs off on your hand as a white powder is chalking. Brief 10 says to wash the chalk off with a detergent solution and a medium soft brush before repainting, because new paint will not hold to it.
How we fit in
Our house wash is a soft wash: low pressure and a professional grade sodium hypochlorite (SH) cleaning solution, given time to work, then rinsed. Sodium hypochlorite is the active ingredient in household bleach, the same chemistry Brief 10 uses on mildew, and the low pressure keeps water out of the wall. Brief 10 warns that water blasting above 600 psi can force water into woodwork and even through the sheathing. Tell your painter when the wash is booked, and leave the wall time to dry before the first coat; your painter and the paint can set the exact wait.
What cleaning will not fix
Washing does not scrape, sand or repair anything. Loose and peeling paint, rotten wood and failed caulk are the painter's prep work, not ours. On older homes, paint may contain lead, and removing it by sanding or scraping can create lead dust, so that is a job for a painter who handles lead safely.
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