What is the proper way to clean exterior siding before painting in Ottawa?

Cleaning is the most underrated step in exterior painting, and in Ottawa's climate it might be the most important one. Our homes collect a unique combination of road salt residue, winter grime, mildew from spring humidity, and pollen that all needs to come off before a single drop of paint goes on.

Why Cleaning Matters So Much in Ottawa

Paint bonds to a surface at the molecular level. Any barrier between the paint and the substrate -- dirt, mildew, chalking from old paint, salt deposits -- weakens that bond. Ottawa exteriors are hit with:

• Road salt spray from October through April. Homes near arterial roads like Carling, Merivale, Baseline, and Hunt Club accumulate heavy salt deposits on lower siding.
• Mildew and algae growth on north-facing walls and under eaves where shade and moisture persist. Ottawa's spring thaw creates weeks of damp conditions that feed mildew.
• Chalking from UV-degraded paint -- that white powdery residue that rubs off on your hand. Ottawa's bright summer sun breaks down exterior paint films over time.
• Pollen accumulation in May and June, particularly from Ottawa's massive tree canopy in neighbourhoods like Rockcliffe, the Experimental Farm area, and Britannia.

If any of this is left on the surface, your new paint job can peel within 1-3 seasons instead of lasting the expected 8-12 years.

Method 1: Pressure Washing (Most Common)

This is the go-to method for most Ottawa exterior cleaning.

Equipment: A pressure washer rated at 1,500-2,500 PSI with a 25-degree fan tip. Never use a zero-degree tip on any residential siding -- it'll gouge wood, crack vinyl, and damage fibre cement. You can rent a quality unit from Home Depot on Innes Road or the Hunt Club Home Hardware for about $75-$100 per day.

Process:

Pre-wet all landscaping near the house to dilute any cleaning solution runoff

Apply a cleaning solution from bottom to top (prevents streaking). More on solutions below.

Let the solution dwell for 10-15 minutes -- don't let it dry on the surface

Rinse from top to bottom at a 45-degree downward angle, maintaining 12-18 inches from the surface

Work in sections no larger than 10x10 feet to prevent the solution from drying

For wood siding: Keep pressure under 1,500 PSI. Wood fibres raise easily and high pressure creates a fuzzy surface that drinks paint. Many Ottawa painters prefer soft washing (see below) for wood.

For vinyl siding: 1,500-2,000 PSI works well. Always spray downward to avoid forcing water behind the lap joints.

For fibre cement (Hardie board): Up to 2,500 PSI is safe. This material is tough but still benefits from a cleaning solution rather than pressure alone.

Method 2: Soft Washing (Preferred for Wood and Older Homes)

Soft washing uses low pressure (under 500 PSI) combined with a stronger cleaning solution to do the work chemically rather than mechanically. This is increasingly the method of choice for Ottawa painters working on:

• Heritage homes in protected districts
• Cedar and pine siding that's soft and easily damaged
• Stucco exteriors common in Alta Vista and Riverside South
• Older homes with fragile caulking that high pressure would blast out

Cleaning Solutions

For general dirt and grime:

• Mix 1 cup TSP (trisodium phosphate) per 4 litres of warm water. TSP costs about $8-$12 per box and is available at any Ottawa hardware store.

For mildew and algae (extremely common in Ottawa):

• Mix 1 part household bleach to 4 parts water, plus a small squirt of dish soap as a surfactant. Apply, let dwell 10 minutes, rinse thoroughly.


• Or use a commercial mildewcide wash like Zinsser Jomax ($15-$20 per concentrate bottle) mixed per label directions.

For heavy chalking:

• TSP solution applied with a stiff bristle brush, working in small sections. Rinse and repeat until the surface no longer leaves powder on a dark cloth rubbed across it.

For salt residue:

• Warm water and TSP are effective. The key is thorough rinsing -- salt dissolves but re-deposits if you don't flush it completely. Ottawa painters doing spring exterior prep often do two full rinse passes on salt-heavy surfaces.

Timing and Drying

After cleaning, the siding needs to dry completely before priming or painting. In Ottawa, this means:

• Summer (June-August): 24-48 hours is usually sufficient
• Spring or fall: 48-72 hours minimum. Morning dew and cooler nighttime temperatures slow evaporation.
• Moisture meter check: Professional painters use a pin-type moisture meter ($30-$50). Wood siding should read below 15% moisture content before paint goes on. Above that, adhesion will fail during Ottawa's winter freeze cycles.

Cost for Professional Exterior Cleaning

Most Ottawa painters include exterior washing in their full repaint quote. As a standalone service:

• Pressure/soft wash of a typical two-storey home: $300-$600


• With mildew treatment: $400-$750


• Full prep wash as part of an exterior repaint contract: often included or discounted to $150-$300

This is 10-15% less than what you'd pay for the same service in the GTA.

Common Mistakes to Avoid

• Don't paint over chalking paint. The new coat will peel. Clean it off completely.
• Don't skip the mildew treatment. Painting over mildew traps it under the film -- it grows through the new paint within a season.
• Don't pressure wash in freezing temperatures. Water in cracks and joints freezes and causes damage. Ottawa's exterior painting season for prep work starts in late April at the earliest.

For professional exterior painters who follow proper cleaning protocols, browse the Ottawa Construction Network directory. And for more questions about prepping your home's exterior, Ottawa Paint Contractors' Paint IQ has you covered.