Why Exterior Painting Starts With a Wash

An exterior wall collects a film you mostly cannot see: dust, pollen, salt carried in off the water, and the chalky powder that old paint sheds as it weathers. Paint applied over that layer is stuck to the layer, not to the house. It can look perfect for a season and then peel away in sheets.

What the wash is actually removing

• Chalking — run a hand down old paintwork and look at your palm. That powder has to come off.
• Green and black growth on the shaded side, which is alive and will grow back through fresh paint if it is only painted over.
• Salt film, which matters on a coastal island more than most places.
• Loose and flaking paint, which the water finds before your brush does.

Pressure is not the point — the right pressure is

A pressure washer can strip paint, and it can just as easily drive water behind siding, carve into soft wood, force it past window seals or blast the sand out of an old mortar joint. Different surfaces take very different pressure. Vinyl siding, painted clapboard, brick, a wood deck and a concrete path are five different jobs, and the wand is angled to wash down and across rather than driven up under the laps where water can get in and stay.

Then it has to dry

Wood that has just been washed is holding water, and paint over damp wood blisters. So a wash is not the day before painting as a formality — the surface needs time to give that moisture back up, and how long depends on the weather that week and which side of the house it is. Rushing this undoes the entire point of having washed.

Washing on its own is a job too

Not everything that gets washed is going to be painted. A path, a patio, a fence, a deck or the shaded side of a house often just needs the growth and grime taken off. It is a fast way to make a property look cared for again, and it is worth doing before you decide whether the paint underneath actually needs redoing — sometimes it does not.