Drying and curing are not the same thing
Render feels dry to touch within a few hours. That is water leaving the surface, and it tells you almost nothing about whether the wall is ready for anything.
Curing is the chemical process where cement reacts with water and gains strength. It continues for about 28 days, and the first seven are where most of the strength develops. During all of that, the render is still releasing moisture out through its surface.
So the question is not really when the render is dry. It is when the render has released enough moisture that you can seal it without trapping the rest inside.
Real Sydney timeframes
Here is what to actually plan around in Sydney conditions.
- Touch dry: 2 to 6 hours depending on temperature and wind
- Second coat can go on: 24 to 48 hours after the scratch coat
- Texture coat can go on: 14 days minimum, 28 days preferred
- Paint can go on: 14 to 28 days, with 28 being the safe answer
- Full cure and strength: around 28 days
What changes those numbers
Weather is the main variable and it moves things in both directions. A dry breezy Sydney spring will have a wall ready at the shorter end. A cold damp winter fortnight with no air movement can push it out well past 28 days.
Thickness matters too. A thick build coat over uneven brickwork holds far more water than a thin coat over flat blockwork, and it takes proportionally longer.
Interior work is often slower than exterior, which surprises people. There is less air movement inside, so moisture leaves more slowly. Running a fan or a dehumidifier genuinely helps.
What happens if you paint too early
You trap moisture behind a film that it cannot get through. It then does one of three things, and often all three.
It pushes the paint off the wall, which shows as blistering and bubbling, usually in the first warm spell after painting. It carries dissolved salts to the surface as it tries to escape, leaving white powdery efflorescence under or through the paint. And because new render is highly alkaline, it can chemically attack the paint binder in a process called saponification, which leaves the paint soft, sticky or discoloured in patches.
None of those are fixable by painting again. The paint has to come off, the wall has to dry properly, and then it has to be repainted. It is a very expensive fortnight to have saved.
How to tell if the wall is actually ready
The professional answer is a moisture meter, and a painter who asks to test before starting is a painter worth keeping.
The version anyone can do is the plastic sheet test. Tape a piece of clear plastic about 400mm square tightly to the wall on all four edges and leave it for 24 hours. If there is condensation on the inside of the plastic, or the render underneath has darkened, the wall is still giving off moisture and it is not ready.
Also look for efflorescence. If white powder keeps appearing on the surface after you brush it off, salts are still migrating out, which means moisture is still moving. Wait.
Curing matters as much as waiting
One thing worth understanding is that render drying fast is not good. Cement needs water to cure, and if a summer sun pulls the moisture out of a fresh wall in a day, the cement never fully hydrates and you end up with a weaker, more crazed render.
That is why we mist fresh render down for the first few days in hot weather, and why we avoid rendering a wall in direct afternoon sun in January if we can sequence around it.
A wall that was cured properly and then given its full 28 days will take paint beautifully and hold it for years. It is worth the wait.