A fresh coat of exterior paint is one of the most impactful upgrades you can make to your home's appearance. But it's also one of the most expensive — and one of the most commonly botched. The most frequent cause of premature paint failure in Ontario is inadequate surface preparation, and pressure washing is the foundation of proper prep. Skip it and you're likely repainting within two years.
Paint Adhesion Science: Why Cleanliness Is Everything
Paint adhesion is a physical and chemical process. Exterior paint bonds to the substrate surface — wood, masonry, stucco, or existing paint — through mechanical adhesion (paint penetrating surface pores and texture) and chemical adhesion (the paint film forming a molecular bond with the substrate).
Both mechanisms are dramatically compromised by contamination between the paint and the surface. Consider what typically accumulates on exterior surfaces over years of Ontario exposure:
- Chalk: Old exterior paint oxidizes and forms a chalky white powder on its surface (you can wipe it off with your hand). Paint applied over chalk adheres to the chalk layer rather than the underlying surface. The chalk then releases, taking the new paint with it.
- Mildew and mould: Living organisms that grow in the pores of the surface. Paint doesn't adhere to biological growth — it sits loosely on top, and as the mould continues to grow, it pushes the paint film away from the surface.
- Dirt and dust: Loose particulates prevent paint from making direct contact with the substrate. The paint bonds to the dirt, which detaches from the surface when exposed to weather.
- Oil and grease: Oils repel water-based paints and prevent adhesion of oil-based paints as well. Even the natural oils in handprints are enough to cause paint adhesion failure.
- Road salt residue: Particularly relevant in Ontario, road salt deposited on siding during winter becomes hygroscopic (draws moisture from the air), keeping siding damp and preventing paint from properly curing and bonding.
A professional painter will not apply paint to a surface that hasn't been properly cleaned. If you're doing your own painting or hiring a painter, the cleaning step is non-negotiable for results that last.
What Happens When You Skip Pressure Washing
The consequences of inadequate cleaning before painting follow a predictable timeline in Ontario's climate:
Within 6–12 months: Paint begins to peel in specific areas — usually where contamination was concentrated. You'll see paint lifting at the edges of boards, bubbling in sun-exposed sections (where heat drives moisture from beneath), and flagging (sheets of paint separating) in areas with mildew.
Within 1–2 years: Peeling becomes widespread. Once paint begins to fail, moisture infiltration accelerates the damage. The underlying wood (if present) begins to grey and weather through the failing paint, and any bare wood sections are now exposed to rot.
Repair costs: Failed paint requires complete removal (stripping, sanding, or heat-gunning) before repainting — significantly more labour than painting clean, properly prepped surfaces. The total cost of a paint job that fails prematurely is often 2–3 times the cost of doing it right the first time.
The bottom line: a professional exterior paint job on a properly prepared surface in Ontario should last 7–10 years. The same job on a poorly prepared surface typically lasts 2–4 years before significant failure.
Pro Tip: Run your hand firmly across the existing paint surface before washing. If white chalk residue transfers to your hand, you have chalky paint. This chalk must be completely removed by pressure washing before priming and painting. A primer specifically designed for chalky surfaces will also help ensure adhesion.
Recommended PSI for Paint Preparation Washing
For paint preparation, you want thorough cleaning without damaging the surface you're about to paint. The target PSI range varies by surface material:
| Surface | Recommended PSI | Tip | Notes |
|---|---|---|---|
| Vinyl and aluminum siding | 1,500–2,000 | 25–40° | Avoid spraying up under panels |
| Wood siding (painted) | 1,000–1,500 | 25–40° | With the grain; high PSI raises wood grain |
| Wood siding (bare/chalky) | 1,500–2,000 | 25° | More pressure needed to remove chalk |
| Brick | 500–1,000 | 40° | Low pressure only; protect mortar joints |
| Stucco (traditional) | Soft wash | N/A | Pressure washes cracks stucco — use soft wash |
| Concrete foundation | 2,000–2,500 | 25° | Standard concrete cleaning approach |
| Trim and fascia | 1,000–1,500 | 40° | Avoid spraying into gaps or window frames |
A wide fan tip (25° or 40°) is appropriate for nearly all paint-prep washing. You're removing loose material and contamination — you don't need the concentrated force of narrow tips, and those tips risk damaging the surface texture you're about to paint.
Surface-Specific Preparation Notes
Wood siding: After pressure washing, inspect for any loose or peeling paint that survived the wash. Scrape these areas with a putty knife — they won't adhere even with proper cleaning. Sand edges of any scraped areas to feather them into the surrounding paint. Apply wood primer to any bare wood before topcoat application.
Brick: Brick to be painted needs thorough cleaning at low pressure. Efflorescence (white salt deposits) on the brick surface must be treated with an efflorescence remover before painting — standard paint will not adhere over it. Brick-specific masonry primer is essential before applying exterior paint to brick.
Stucco: Traditional stucco must be cleaned by soft washing — never by pressure washing. Even low pressure can create hairline cracks in stucco that become water infiltration points. After soft washing, inspect for existing cracks and hairline fractures. Fill with exterior acrylic caulk before painting. Use elastomeric masonry paint for stucco — it bridges hairline cracks and handles the expansion and contraction of the stucco.
Chalky paint surfaces: Identify chalky sections (white powder on your hand when you wipe the surface). These require aggressive cleaning — a 25° tip at 1,500–2,000 PSI to remove the chalk layer. After washing, when dry, run your hand across the surface again. If chalk is still present, re-wash that section. Apply a chalk-binding primer before topcoat.
Mildew-affected surfaces: For surfaces with significant mould or mildew, combine pressure washing with a cleaning solution containing sodium hypochlorite (1–2% concentration). Apply the solution, let dwell, then pressure wash. Mildew-resistant exterior paint is available and worth using on problem areas — north-facing walls and under-eave sections particularly.
"We've seen homeowners spend $8,000 on a paint job only to watch it peel within 18 months because the painter skipped the pressure wash. Proper prep is the difference between a 2-year paint job and a 10-year paint job."
— David, D&D Home Services Co-Founder
Drying Time: The Critical Waiting Period
After pressure washing, the surface must dry completely before painting begins. This is one of the most-overlooked requirements in exterior painting preparation.
Minimum drying time: 48–72 hours after pressure washing in typical Ontario spring and summer conditions. Many professionals recommend waiting a full week, particularly for wood siding.
The consequences of painting over damp wood are severe: paint blisters and peels as trapped moisture tries to escape through the paint film. Once moisture is trapped under paint, the damage is ongoing — the wood continues to wet and dry with humidity changes, and each cycle degrades the paint adhesion further.
Factors that affect drying time:
- Temperature: Cold Ontario spring weather slows drying significantly. Below 10°C, wood doesn't dry effectively and paint doesn't cure properly either — most exterior paints have a minimum application temperature of 10°C.
- Humidity: High humidity slows evaporation. Avoid painting during or immediately after rain periods. Check the 5-day forecast before scheduling.
- Sun exposure: South and west-facing walls dry faster than north and east-facing walls. On a single house, different elevations may be ready on different days.
- Wood species and age: Dense hardwoods and aged, weathered wood absorb and hold more moisture than newer or lighter wood. Allow extra drying time for heavily weathered siding.
Use a moisture meter to verify wood moisture content before painting — look for readings under 15% for best results. This removes the guesswork from determining if enough time has passed.
Professional Prep Service: When to Hire It Out
Many painters offer power washing as part of their preparation service. However, not all painters are exterior cleaning specialists — some do a cursory rinse rather than a thorough cleaning. If you're hiring a painter, ask specifically:
- What PSI will you use for washing?
- Will you treat for mildew as part of the washing?
- How many days between washing and painting?
- Will you use a moisture meter before painting begins?
Hiring a dedicated exterior cleaning company for the prep wash, separate from the painting contractor, ensures the cleaning step receives proper attention. Our pressure washing service is frequently used by both homeowners and painting contractors throughout Kitchener-Waterloo as the preparation step before exterior painting.
The cost of professional pressure washing for paint prep on a typical Kitchener-Waterloo home runs $150–$400 depending on size and condition. Compared to the cost of a paint job that fails early and requires redoing, this investment is straightforward arithmetic.
Key Takeaways
- ✓ Paint adheres to contamination, not the substrate — chalk, mildew, and dirt all cause premature peeling
- ✓ A good paint job on dirty siding lasts 2 years; the same job on clean, dry siding lasts 7–10 years
- ✓ 1,500–2,000 PSI with a 25–40° tip is appropriate for most siding paint prep
- ✓ Stucco must be soft-washed, not pressure-washed before painting
- ✓ Wait 48–72 hours minimum after washing before any paint is applied
- ✓ Mildew must be chemically treated, not just rinsed away, for paint to adhere long-term
