The short answer
Paint bonds only to what it touches — wash-skipped chalk, mildew and dirt become the layer your new paint peels off with. Pre-paint washing means cleaning, killing mildew with a proper solution, and rinsing, then allowing 24–72 hours of dry weather before primer. Homes built before 1978 need lead-safe practices, not aggressive blasting.
Why Good Paint Fails Early
When a paint job peels in three years instead of lasting twelve, the autopsy almost never blames the paint. It blames the layer under it. Paint bonds to whatever it touches — and if that is chalked old finish, mildew, pollen film or dust, then the expensive new coating is attached to a weak intermediate layer that lets go on its own schedule.
Mildew is the worst case: paint over live mildew and it keeps growing, pushing through the new film as gray-black staining within a season or two. No primer fixes biology.
What Pre-Paint Washing Actually Involves
- Kill the growth. A soft-wash solution treats mildew and algae at the root — dead organisms rinse away; live ones return through the paint.
- Cut the chalk. Oxidized paint sheds a powder (rub a hand on old siding and it comes back white). A surfactant wash removes the loose chalk so primer meets sound film.
- Rinse thoroughly at low pressure. The goal is a clean substrate, not a stripped one — blasting is not prep.
- Let loose paint declare itself. A proper wash lifts paint that was already failing. That is the wash doing your scraping crew's triage for free — those areas get scraped, feathered and spot-primed.
- Dry. The step everyone rushes, and the one that ruins jobs.
How Long to Wait Before Painting
| Surface | Minimum dry time after washing |
|---|---|
| Vinyl / aluminum (repaint) | 24 hours of dry weather |
| Wood siding and trim | 48–72 hours — wood holds water in its grain |
| Brick / masonry before coating | Several dry days; masonry absorbs deeply |
| After any rain during the wait | Restart the clock |
Painting over damp substrate traps moisture behind an impermeable film — blistering and peeling are the guaranteed result, usually within the first hot season. In a Michigan spring, building the dry window into the schedule is the difference between a paint job and a redo.
Homes built before 1978: assume lead paint until tested. High-pressure blasting of failing lead paint scatters contaminated chips and dust across the property — the EPA's RRP rules exist for exactly this. The correct prep is gentle washing plus contained scraping by an RRP-certified painting contractor. Any washer who wants to "blast it all off" a pre-1978 home is proposing a hazard, not a service.
Wash Even If You Are Not Repainting Everything
Trim-only repaint? The whole elevation still benefits from washing first — paint edges bond where old meets new, and a uniform clean surface is what makes touch-up blend instead of outline itself. Painters quote tighter on clean houses too: what they can see, they do not pad for.
Who Does What
The clean division of labor: we wash the house — growth killed, chalk cut, surfaces rinsed — and the painting contractor owns scraping, sanding, priming and coating. Some painters wash themselves; plenty prefer arriving to a clean dry house with the failing areas already revealed. Coordinate the schedule so the dry window lands between us and them, with weather margin.
The Bottom Line
Washing is not a nicety before painting — it is the step that decides whether the paint's warranty years actually arrive. Kill the growth, cut the chalk, rinse gently, then respect the drying clock. The cheapest insurance on a four-figure paint job is the wash that costs a fraction of one redo.
Want This Handled Properly?
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