Rendered walls look solid and inert, so when the paint on one starts to bubble or peel away in patches, the obvious suspect is the paint. Usually it is not. Render is a masonry skin that takes on water and gives it back, and the coating on top decides how easily the wall can dry. Put the wrong system on a wall that is regularly wet and the water collects behind the film until it pushes it off. Getting this right is less about buying a better tin and more about reading the wall first.
Call Jimmy — 0432 077 782Cement render over brick absorbs rain on the weather side, draws moisture up from the ground at the base of the wall, and picks up water wherever a gutter overflows or a sill sheds onto it. None of that is a defect on its own. A healthy wall takes water in and lets it out again, mostly as vapour leaving through the face as the sun warms it.
That last part is the one that matters for paint. Every coating slows the rate at which a wall can dry through its surface, some far more than others. On a wall that stays fairly dry, it makes no practical difference. On a wall that is wet for a large part of the year, the coating becomes the bottleneck, and whatever cannot get out through it builds up underneath.
Render membranes are thick, elastic coatings. Their strength is movement: they stretch across the fine hairline cracks that open and close in cement render with the seasons, so those cracks stop showing through and stop letting rain in. On sound, dry render with that kind of cracking, a membrane is often the right answer and it does a job ordinary exterior paint cannot.
The same thickness that makes a membrane good at bridging cracks makes it slower for the wall to dry through. If water is getting in from behind or below, rather than from the face, a membrane holds it in. The symptoms are soft bubbles that sometimes hold water when pressed, patches that lift low on the wall while the upper wall stays sound, and render that has gone powdery under a coating that still looks intact from the street. People then blame the membrane, when the real fault was choosing it for a wall that could not take it.
Render on older Melbourne homes, particularly Victorian and Edwardian ones, is often softer and more lime-based than modern cement render. Lime render is designed to let moisture through it freely. Seal it under a tight modern coating and the water and salts that used to evaporate from the face are trapped in the render instead, which slowly breaks it down from the inside.
Older brick homes also tend to have damp-proof courses that have failed, or that have been bridged over the decades by paving, concrete paths or garden beds built up against the wall above them. Ground moisture then rises into the masonry and has to leave somewhere. A white, powdery bloom of salt near the base of the wall, or a tide-mark band at a consistent height, is the wall telling you that is happening.
Before anything is specified, the wall needs walking with the moisture question in mind. Look low first, because rising damp and splash-back both show near the ground. Check whether paving or garden beds sit above the line where the damp course should be. Tap the render: a hollow sound over an area means it has come away from the brick behind it. Look at every gutter, downpipe, sill and parapet cap above a failed patch, because the water source is usually directly above or below the damage.
It also helps to note which elevations are failing. Paint lifting on one wall only, under a gutter that overflows, is a water problem with a paint symptom. Paint failing low on every side of an older house points at ground moisture. Both need their cause dealt with before any coating choice matters. On period homes we inspect for render cracks and rising damp before specifying a paint system, because a coating chosen without that information is a guess.
The order is always the same. Stop the water getting in: clear and repair the gutters, redirect downpipes that discharge against the wall, lower or pull back garden beds and paving that bridge the damp course, and repair cracked sills and cappings. Then give the wall time to dry out before judging it again. Failed coating comes off, salt deposits are brushed off dry rather than washed back into the wall, and loose or hollow render is repaired.
Only then does the coating decision make sense. Sound, dry cement render with hairline cracking suits a membrane. Render that will always see some moisture, and soft lime-based render on older homes, needs a more vapour-open system that lets the wall keep breathing even if it costs some crack-bridging. A painter who quotes a membrane for every rendered wall without asking where the water is coming from is selling the same answer to different problems.
Almost always because moisture is collecting behind the coating and pushing it off. The water is coming from somewhere: rain getting in through cracks or a failed sill, a gutter or downpipe soaking the wall, or ground moisture rising through older brickwork. The coating may have made it worse if it is a tight, thick film that slows the wall's drying, but the water is the cause.
On sound, dry cement render with fine seasonal cracking, a membrane usually performs better because it stretches across the cracks. On a wall with an ongoing moisture problem, or on soft lime-based render on an older home, a membrane can trap water in the wall and fail sooner than a more vapour-open coating would. The right answer depends on the wall, not the product.
Not successfully. That powder is salt carried out of the masonry by water, and it keeps coming as long as the water does. New paint over it loses adhesion and is pushed off as more salt forms behind it. The salt is brushed off dry, the source of the moisture is found and fixed, and the wall is checked again once it has dried.
Long enough to cure fully, which is set by the render manufacturer and the weather rather than by a rule of thumb. New render still holds a great deal of water from mixing, and coating it early seals that water in. Painting green render is one of the more common reasons a brand-new rendered wall blisters in its first summer.
No, but paint will not fix it. The damp has to be addressed first, often by removing whatever is bridging the damp-proof course, such as built-up garden beds or paving, or by having the damp course itself repaired. Once the wall has dried, a coating that suits a wall which may still see some moisture gives the repaint a far better chance of lasting.