There is a version of exterior painting that costs less and takes half as long: show up, roll two coats over whatever is there, and leave. It looks excellent for about eighteen months. Then it starts letting go in sheets, because paint bonds to what it touches — and what it touched was dirt.
What is actually on your siding
An exterior that has not been washed in several years is carrying more than it looks like it is.
- Chalk — the powdery residue of the old coating breaking down under UV exposure
- Mildew and algae, particularly on north-facing and shaded elevations
- Pollen, road dust, and general atmospheric grime
- Loose and flaking paint that is already lifting at the edges
Chalk is the one that ruins paint jobs
Run your hand along old painted siding. If it comes away powdery, that is chalk — the binder in the old paint has degraded and the pigment is releasing. New paint applied over it bonds to a loose powder layer rather than to the substrate.
It will look perfect on the day. Then the first serious freeze-thaw cycle takes the whole coating off in sheets, and the failure edge shows a clean powder line underneath.
Mildew grows back through paint
Mildew is alive. Painting over it does not kill it — it continues growing under the film and pushes back through, showing as dark spotting within a season or two. It has to be treated with the appropriate solution and rinsed off, not sealed in.
Washing is not the same as blasting
This is the part that matters, and it is where a lot of damage gets done. Siding should be soft washed — low pressure, with a cleaning solution doing the actual work and dwelling long enough to break down what is on the surface.
High pressure aimed at siding drives water up behind the laps and into the wall assembly, which is precisely the opposite of what siding exists to prevent. On wood it also raises the grain and leaves permanent wand marks. Pressure is a tool for concrete and masonry, not for the side of your house.
Then let it dry
Washing is only half of it. The surface has to dry fully before prep begins, and how long that takes depends on material, exposure, and weather. Painting damp siding traps moisture behind the film and causes blistering — a different failure with the same result.
The full sequence is wash, dry, scrape and sand back to sound material, spot-prime bare wood, caulk the joints, and only then coat. Skip any step and you are buying a shorter paint job than you paid for.