Render is not a normal surface
Three things make a rendered wall different from anything else you might paint, and all three catch people out.
It is strongly alkaline. Fresh cement render has a pH up around 12 or 13, which is aggressive enough to chemically attack the binder in an ordinary paint. That reaction is called saponification and it leaves the film soft, sticky or discoloured in patches, sometimes months later.
It is porous. Bare render drinks the first coat unevenly depending on how well each area was floated, which is why an unsealed wall can look patchy no matter how many top coats go on top.
And it keeps releasing moisture for weeks after it feels dry to touch. Seal that moisture in behind a film and it pushes the paint off the wall.
What to actually use
For a new rendered wall the system is a sealer plus two top coats, and the sealer is the part people skip.
- Alkali resistant sealer or primer, made specifically for masonry and render. This is what protects your top coats from the alkalinity still in the wall
- Two coats of quality exterior acrylic. Acrylic stays flexible, which matters because render moves with temperature, and it lets water vapour out while keeping wind driven rain from getting in
- Avoid oil and solvent based enamels on render. They are far less breathable, so any moisture in the wall has nowhere to go and lifts the film
- Near the coast, use a product rated for that exposure. Salt is the leading cause of coating failure from Bondi through to Cronulla
Breathability is the thing to understand
This sounds like marketing language and it is genuinely the most important property of a coating on masonry.
You want a film that stops liquid water getting in from outside while still letting water vapour escape from inside the wall. Masonry always holds some moisture, and it always needs a route out.
Put a fully sealed impermeable coating on a rendered wall and you trap that moisture against the render. It then does what trapped moisture always does, which is find the weakest point and push. That shows up as blistering, or as render debonding from the wall behind an intact looking paint film.
This is exactly why a cheap general purpose exterior paint on a render job so often fails early. It is not always that the paint is bad, it is that it was the wrong type of film for a wall that needs to breathe.
Painting over old render that has already been painted
Different job, and usually easier. An older wall is fully cured, so alkalinity is no longer the issue, and you generally do not need a sealer.
What you do need is to check two things. First, chalking. Rub your hand across the wall. If it comes away with a powdery residue, the old coating has broken down under UV and new paint applied over it will take that powder with it when it eventually lets go. That wall needs a binding primer.
Second, sound the render. Tap systematically across the wall. A solid dull note means it is bonded. A hollow note means it has let go from the masonry behind. Paint does not hold failing render together, so anything drummy has to come off and be made good first.
The alternative worth knowing about
If your wall is being rendered anyway, you do not have to paint it at all. A tinted texture coating is coloured right through the material rather than filmed on top, so there is no paint layer to blister, peel or fade unevenly.
It costs more on the day. Over twenty years it usually costs less, because you apply it once instead of repainting two or three times, and you have scaffolding or ladders up on the house once instead of three times.
Paint still makes more sense if you like changing colour, or if the budget only stretches so far right now. Both are legitimate. It is just worth making the choice deliberately rather than defaulting to paint because that is what everyone does.