By EA Paint & Light Carpentry7 min read
At a glance
- Dry before paintAverage
- At least a day or two of dry weather
- Safer for siding
- Soft washing, low pressure
- Pressure risk
- Water forced behind siding
- Mildew
- Needs a cleaner that kills it
Times are rough averages for planning only. Every house is different — size, condition, repairs and weather all change it. Your written estimate gives the actual schedule for your job.
What is the difference between soft washing and power washing siding?
Power washing uses high-pressure water to blast dirt off. Soft washing uses low pressure with a cleaning solution that kills mildew and algae, which is then rinsed away. For most house siding, soft washing is the safer choice, because high pressure can force water behind siding, gouge wood, strip loose old paint and damage windows.
What is soft washing, and what is power washing?
Soft washing applies a cleaning solution to the siding at low pressure, lets it work for a few minutes, and rinses it off at low pressure, often not much stronger than a garden hose. The solution does the cleaning. Most soft-wash mixes for siding are based on diluted sodium hypochlorite, the active ingredient in household bleach, with a surfactant to help it cling. They kill the mildew, algae and lichen that make a wall look dirty, rather than just knocking off the top layer.
Power washing, or pressure washing, drives water at high pressure through a narrow nozzle. It is good at removing dirt and grime from hard surfaces such as concrete, paving and sound masonry. On siding it relies on force, and force is the problem: the jet that cleans a sidewalk can cut wood, lift paint and push water where it was never meant to go. Strictly, a power washer heats the water and a pressure washer does not, but most people use the terms interchangeably.
Pressure
- Soft washing
- Low, close to a garden hose
- Power washing
- High
How it cleans
- Soft washing
- Solution kills and loosens growth
- Power washing
- Force of the water strips dirt off
Best for
- Soft washing
- Vinyl, painted wood, cedar shingles, fiber cement
- Power washing
- Concrete, paving, sound masonry
Mildew and algae
- Soft washing
- Kills the growth
- Power washing
- Removes what is visible; growth often returns sooner
Main risks
- Soft washing
- Plant damage if the solution is not managed
- Power washing
- Water intrusion, gouged wood, stripped paint, broken seals
How high pressure damages siding and windows
High pressure is not wrong everywhere, but on a house it causes damage that often does not show until later:
- Water forced behind siding: vinyl laps and clapboard joints are built to shed rain falling down, not water sprayed upward under pressure, which can soak the sheathing, insulation and wall cavity
- Gouged and furred wood: cedar and pine are soft, and a narrow jet can etch lines, raise the grain and leave a fuzzy surface that holds paint poorly
- Damaged paint: pressure can strip sound paint and drive water under the edges of paint that is still attached
- Windows: the jet can crack glass, damage the seals on insulated windows and tear screens
- Water indoors: around window and door trim, vents, light fixtures and outlets, high pressure finds every gap
Older homes and lead paint
On houses with old paint, the washing method is also a health question. Paint applied before 1978 may contain lead, and high-pressure water turns loose old paint into chips and contaminated water that spread across lawns, garden beds and play areas. In Northborough, half of homes were built before 1980, and in Marlborough and Hudson the figure is 60 percent, so many houses in the area may have older paint somewhere under the current coat.
If your house may have lead paint, ask any contractor who will wash or prepare it how they will contain and collect paint chips and wash water, and ask for their lead-safe renovation credentials. Massachusetts runs its own lead-safe renovation program through the Department of Labor Standards. A paint test tells you what is actually on the house.
Mildew and algae on north-facing siding in New England
The green, grey or black film that builds up on the north side of a Massachusetts house, behind shrubs and under trees, is mostly mildew and algae. Those walls get little direct sun, dry slowly after rain and dew, and stay damp through humid summers — everything the growth needs. Pressure can remove what you see, but growth left in the texture of the siding often returns within a season or two.
A cleaner that kills the growth deals with the cause. Painting over mildew without killing it first is a common reason new paint shows dark spots again within a year or two.
Tip: To tell mildew from dirt, dab a little diluted household bleach on a dark spot. If it lightens within a minute or two, it is mildew.
Protecting plants and property during soft washing
The solutions that kill mildew on siding can also burn plants and discolor some surfaces. A careful soft wash includes:
- Soaking shrubs, flower beds and lawn near the house with plain water before and after washing, so less solution is absorbed
- Covering delicate plants, and mixing the solution only as strong as the job needs
- Rinsing the siding thoroughly so solution does not drip onto beds for hours afterward
- Closing windows, covering outlets and exterior light fixtures, and moving cars, furniture and toys out of the way
- Keeping runoff away from ponds, wells and storm drains where possible
Washing before painting: how long to let the house dry
A house is washed before it is painted because paint does not hold well on dirt, chalky old paint, pollen or mildew. The usual order is wash, let dry, then scrape, sand, repair and prime, then caulk and paint. Washing first means the scraping happens on clean, dry surfaces, and the painter can see what is really failing.
How long to wait depends on the weather and the siding. Plan on at least a day or two of dry weather, and longer for bare or weathered wood, shaded walls and cool, humid spells. Wood that feels dry on the surface can still hold moisture underneath; a painter can check with a moisture meter, and paint labels often say how dry the surface should be. Painting damp wood is one of the most common causes of blistering and peeling.
Can you soft wash a deck?
Decks and fences are wood too, and the same caution applies. A pressure washer held too close, or fitted with too narrow a tip, cuts into soft pressure-treated pine and cedar and raises fuzzy fibers that have to be sanded off before stain will go on evenly.
Deck cleaning usually combines a cleaner or brightener made for wood with low to moderate pressure, a wide fan tip held well back from the surface, and rinsing along the length of the boards. Removing an old solid stain is a different job, done with a stripper rather than more pressure. Either way, the wood then needs to dry fully before it is stained.
What to ask before someone washes your house
Ask what method will be used on each surface and at what pressure, which cleaning solution will be used, and how plants will be protected. On an older house, ask how paint chips and runoff will be handled. If painting follows, ask how long the house will dry before prep starts. If someone plans to use high pressure on painted siding, ask why, and what they will do about water behind the siding and around the windows.
On a painting estimate, washing should appear as part of the preparation, with the method named, rather than a single line that says power wash and nothing more.
Talk to a local crew
EA Paint & Light Carpentry is a painting and light carpentry company based in Northborough, MA. If you would like the house looked at, call (508) 466-5846 or request a free written estimate. Related services: Exterior Painting, Power Washing & Deck Staining.
