Why is my masonry paint peeling off the wall?

Masonry paint peels off render for one of three reasons, and only one of them is about the paint. How to tell which you have before spending a summer on it.

Difficulty: advanced
Time: 2 hr
Read: 5 min
Safety: warning

Short answer

Either the wall is wet and cannot dry through the coating, the render or the previous coating underneath was never sound, or the surface was chalking and the new paint bonded to dust. Look at the back of a flake and at where on the building the failure is — both tell you which.

Repainting a failed elevation is a large, expensive job at height, and doing it twice is unbearable. The three causes look similar from the ground and are completely different underneath, so the diagnosis is worth an hour before the scaffolding goes up. The most serious of the three is not really a paint failure at all: it is a wall that has been sealed and can no longer dry.

Safety

This work is at height on ladders or scaffolding, often over hard ground. Use a tower or scaffold rather than a ladder for anything more than a small area, and never lean sideways from a ladder to reach. Paint on a house predating the 1990s may contain lead: do not dry-sand or burn it off, and do not let debris blow into a garden used by children or a vegetable patch.

What you'll need

Safe access — a tower or scaffold rather than a ladder
A scraper and a stiff brush
A masonry stabilising solution
The correct masonry system for the wall, with its own primer
A moisture meter

Why an impermeable coating blows off a wall that needs to dry outwards

Step by step

Look at the back of a flake first.Clean paint on the back means the coatings separated. Render, sand or grey powder on the back means the render surface came away, which is a substrate problem. A smooth shiny back means it never bonded. This takes a minute and narrows everything that follows.
Map where the failure is on the building.Failure concentrated low down suggests rising or splashing moisture and a possible damp-proof course problem. Failure below a window, under a sill or beside a downpipe suggests water running where it should not. Failure on the whole of one elevation, usually the one facing the prevailing weather, suggests driven rain and a coating that cannot let the wall dry. Failure everywhere evenly suggests the coating or the preparation.
Test whether the wall can breathe.If the wall is solid rather than cavity-built, or the render is lime, and the existing coating is a plastic or heavily filled masonry paint, you have the classic case: water gets into the wall, cannot get out through the film, and lifts it. Frost then shears film and render face off together. No amount of recoating with the same type of product will fix this.
Check for chalking under the failed film.Rub the exposed surface. If it powders heavily, the previous coating had degraded and the failed layer was bonded to dust rather than to the wall. That is a preparation failure and it is fixable.
Fix the water before the coating, always.Clear and repair gutters and downpipes, repoint failed mortar, check that sills throw water clear and that their drip grooves are not bridged, and check that ground level and paving have not risen above the damp-proof course. Nothing you paint on the outside survives a wall that is being fed water.
Strip back further than you think, then stabilise.Scrape until the coating genuinely will not lift anywhere. Repair hollow or cracked render properly. Brush the whole surface down, then apply a masonry stabilising solution to bind any residual chalk and give a uniform key.
Choose the system for the wall, not the shelf.On a solid or lime-rendered wall, a genuinely vapour-permeable mineral, silicate or breathable system with its own primer. On a modern cavity wall with sound render, a good acrylic masonry paint. Buy the primer and topcoat from the same range and follow the maker's sequence.
Accept when stripping everything is the answer.Where an impermeable coating has sealed an old solid wall, the real repair is removing it entirely and letting the wall dry before recoating with a breathable system. It is a big job and it is the only one that lasts.

Tips

A wall that has been fine for decades and starts failing a few years after being painted is telling you the coating changed something. Believe it.
Do a test patch through a full winter before committing to a whole house. One elevation stripped, stabilised and recoated correctly, then inspected in spring, is cheap insurance.
Ivy, dense planting and raised borders against a wall all keep it wet and are a common contributor. Cutting them back does more than another coat of paint.

Common mistakes

Recoating with the same impermeable product — The trapped moisture is still trapped. The new film blisters and sheets off exactly as the old one did, and you have spent a summer at height for nothing.
Painting over a wall with internal damp patches — The coating hides the symptom and prevents the wall drying outwards, so the internal problem gets worse rather than better.
Pressure-washing render hard to remove loose paint — It drives water deep into the wall and can damage the render face. Use a modest setting, or scrape, and give the wall a long time to dry afterwards.

If it doesn't work

Large sheets lifting on one elevation, worse after wet weather

Cause: Impermeable coating on a wall that can only dry outwards — Fix: Strip the coating, repair the render, let the wall dry, and recoat with a genuinely vapour-permeable system.

Paint coming away with the render face attached

Cause: The render surface has failed, not the paint — Fix: Cut out and repair the failed render properly, allow it to cure, prime with an alkali-resisting primer and then coat.

Failure only in a band low down on the wall

Cause: Rising or splashing moisture, or a bridged damp-proof course — Fix: Check ground and paving levels against the damp-proof course, clear any bridging, and address the moisture before recoating.

Paint flaking off in small chips everywhere with powder underneath

Cause: Applied over a chalking surface without stabilising — Fix: Brush and wash the whole elevation, apply a masonry stabilising solution, then recoat.

Questions people ask

Can I just paint over peeling masonry paint?

No. Anything that will lift has to come off, and the reason it lifted has to be addressed. Painting over it buys a season at best.

Is breathable masonry paint worth the extra cost?

On a solid or lime-rendered wall it is not an upgrade, it is the requirement. On a modern cavity wall with sound render, a good acrylic masonry paint is fine.

Should I paint my rendered house at all?

If it is already painted, it needs recoating periodically forever. If it is unpainted render or bare brick and performing well, leaving it alone is often the better decision.