How salt attack works
Airborne salt settles on a wall constantly if you live near open water. Rain and humidity dissolve it, and the salt solution soaks into the pores of the render and the masonry behind it.
Then the wall dries. As the water evaporates, the dissolved salt has nowhere to go, so it crystallises inside the pores. Salt crystals occupy more volume than the solution did, so they exert pressure outwards from inside the material.
One cycle does nothing measurable. Thousands of cycles over years progressively break the material apart from the inside. The technical name is salt crystallisation damage, and it is why a coastal wall can look perfectly sound from the street and come away in sheets when you tap it.
What it looks like
Salt damage has a fairly distinctive set of symptoms.
- White powdery deposits on the surface, called efflorescence, which reappear after you brush them off
- A sandy, friable surface that rubs away under a finger
- Blistering and flaking, often concentrated in a band rather than evenly spread
- Render sounding hollow across broad areas while still looking intact
- Damage worst on the elevation facing the water and on the lower part of walls
Where it is worst in Sydney
As a rough rule, anything within about five hundred metres of open surf is a genuine coastal exposure environment. That takes in most of Bondi, Bronte, Coogee, Maroubra, Manly, Freshwater, Curl Curl, Collaroy, Narrabeen and Cronulla.
Sheltered water is milder but not negligible. Properties around Botany Bay, the Georges River, Sydney Harbour and Pittwater still pick up meaningful salt load, just at a slower rate.
Exposure direction matters as much as distance. A wall facing directly into the prevailing southerly or north easterly gets far more salt deposited on it than a sheltered elevation twenty metres away on the same building.
Why plain cement render struggles
Cement render is relatively porous, which is normally a virtue because it lets a wall breathe. In a coastal environment that porosity is also the pathway salt uses to get inside the material.
It is also rigid, so as salt crystallisation pressure builds inside the pore structure, there is no give. The material simply fractures at a microscopic level, and eventually at a visible one.
None of that means cement render cannot be used near the coast. It means it should not be the final exposed surface there.
What actually works
A polymer modified acrylic system is the practical answer, and it helps in three separate ways.
It is far less permeable, so much less salt solution gets into the material in the first place. It is flexible, so what pressure does build up is absorbed rather than fracturing the surface. And it is more UV stable, which matters because coastal walls also take a heavy sun load.
Crucially, a good acrylic system is still vapour permeable outward. That is not a contradiction. You want a coating that keeps liquid water and salt solution out while letting water vapour escape from inside the wall. A fully sealed impermeable coating traps moisture behind it and makes everything worse.
The details that matter more near the water
Coastal jobs are won and lost on the details, not the flat wall.
- Parapet tops sealed properly, because an unsealed parapet lets salt laden water straight into the top of the wall
- Window sills with a genuine fall away from the building and a drip edge underneath
- Render stopped clear of the ground rather than bridging into soil or paving
- Mid tone colours rather than brilliant white, because efflorescence is far less visible and the wall stays presentable longer between washes
- A freshwater hose down of the exposed elevations a couple of times a year, which sounds trivial and genuinely extends the life of the coating
If the damage is already there
Recoating over salt damaged render does not work. The salt is already inside the material and it keeps cycling, so a new coating over the top fails from behind, usually faster than the original did.
The affected render has to come off. The masonry underneath then needs washing down with fresh water and drying out, and on badly affected walls a poultice can be used to draw salt out of the substrate before recoating.
It is more work than people hope for when they call. It is also the only version that lasts, and on a coastal wall the difference between doing it properly and doing it cheaply shows up within about three years.