Nearly every one of those is the same underlying mistake. The render was painted before it was ready.
Why timing decides everything
Render feels dry within hours. That is water leaving the surface and it tells you very little. Underneath, cement keeps chemically curing for around 28 days, and throughout that period the wall is still pushing moisture outward and is strongly alkaline.
Seal that in behind a paint film and three things happen, often all at once. Trapped moisture pushes the paint off the wall, which shows as blistering. Dissolved salts migrate to the surface and leave white powdery efflorescence under or through the colour. And the high alkalinity chemically attacks the paint binder in a reaction called saponification, which leaves the film soft, sticky or discoloured in patches.
None of that can be fixed by painting again. The paint has to come off, the wall has to dry properly, and the job has to be done a second time.
How long to actually wait
Allow 14 to 28 days in Sydney conditions, and treat 28 as the safe answer. Cold or damp weather pushes it out further, and a thick build coat over uneven brickwork holds far more water than a thin coat over flat blockwork.
- Touch dry: a few hours, and means nothing
- Second render coat: 24 to 48 hours after the first
- Minimum before painting: 14 days in good drying conditions
- Safe before painting: 28 days
- Full cure: around 28 days
The test anyone can do
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 watch for efflorescence. If white powder keeps reappearing on the surface after you brush it off, salts are still migrating out, which means moisture is still moving through the wall. Wait.
A painter who asks to moisture test before starting is a painter worth keeping.
What to put on it
New render usually wants an alkali resistant sealer first, then two top coats of a quality exterior acrylic. The sealer is the part that protects the top coats from the remaining alkalinity in the wall, and skipping it on young render is asking for saponification.
On older render that is sound and previously painted, you generally do not need a sealer, just a proper wash down, scraping of anything loose, and a check for chalking. Rub a hand across the wall. If it comes away powdery, the old coating has broken down under UV and it needs a binding primer or it will take your new paint off with it.
One thing worth knowing before you spend the money: if you are rendering anyway, a tinted texture coating is coloured right through and never needs painting. It costs more up front and saves you a repaint every seven to ten years.
Painting over old, failing render
Paint will not hold a failing render together and it will not stop a crack. If sections of the wall sound hollow when you tap them, the render has lost its bond and painting over it just puts a fresh coat on something that is coming off anyway.
Sound the wall first by tapping systematically across it. Solid dull note means bonded. Hollow note means it has let go. Anything drummy needs to come off and be made good before any coating goes on.
Because we do the render as well, we can deal with both in one visit rather than you paying a painter to discover the problem and then having to find a renderer.