Paint fails for three reasons: it was applied over something loose, over something living, or over moisture. A good wash addresses the first two. The third is a timing problem that washing makes worse before it makes it better.
Read this before you start: lead paint
If the building was painted before 1978, assume lead paint until tested. Pressure washing lead paint aerosolizes it and spreads chips and dust across your garden, your neighbor’s garden and anywhere children play.
This is a genuine health matter rather than a cautious footnote. Test kits are inexpensive and widely available, and if lead is present the surface preparation is a job with specific containment requirements — not a weekend with a pressure washer. The EPA publishes guidance on renovation work involving lead paint; read it before deciding anything.
What paint actually needs
No loose material. Flaking paint, chalking, powdery oxidation. Paint bonds to whatever it touches, and if that is a loose layer it fails with it. Washing removes some of this; scraping and sanding removes the rest.
No living growth. Mildew under paint keeps growing and pushes the coating off. This needs a cleaner, not pressure — a house wash with real dwell time.
No chalk, grease or residue. Including cleaner residue, which blocks adhesion exactly as effectively as dirt. Rinse thoroughly.
Dry substrate. The one that decides most paint failures.
Drying: longer than for staining
Paint is less tolerant of trapped moisture than stain, because it forms a film that moisture then has to push through. Expect a minimum of two dry days and plan on three to four for wood, longer after heavy rain or in humid weather.
Masonry and fiber-cement hold water longer than people expect, particularly on a shaded elevation. A moisture meter is the only way to know rather than guess, and your paint will state a maximum substrate moisture content — that figure takes precedence over anything here.
Pressure that damages, not prepares
Too much pressure on wood raises the grain, and painting over furred wood gives a rough finish that holds dirt and fails sooner. On siding it can drive water behind the laps, where it sits behind your fresh paint and pushes it off months later.
So the preparation wash is a soft wash: cleaner, dwell, gentle rinse with a wide tip. The reasoning is in soft wash vs pressure wash, and the siding-specific method in how to pressure wash vinyl siding.
The sequence
- Test for lead if the building predates 1978. Stop here if positive.
- Scrape and sand all loose and flaking paint. Washing is not a substitute for this.
- Repair any rot, failed caulking and gaps.
- Pre-wet planting, apply a cleaner, dwell five to ten minutes, rinse gently with a wide tip.
- Rinse thoroughly — all cleaner off, especially out of laps and crevices.
- Let it dry. Test with a meter rather than assuming.
- Prime bare areas, then paint, following the product’s own requirements.
When washing is not needed
A recently built, visibly clean surface with no growth and no chalking. Washing adds days of drying for no adhesion benefit, and on new timber it risks furring.
A dust-off and a wipe with a damp cloth is sometimes the correct preparation, and the time saved is better spent on scraping and priming — the steps that actually decide how long the paint lasts. The deck version of this argument is in pressure washing before staining a deck.
