Rendered walls look simple, but render behaves differently from timber or plasterboard. It is alkaline when new, it can crack as it dries, and it can push white salts to the surface. Paint applied without allowing for that can stain, blister or peel. Here is what to check before painting render, whether it is a new rendered extension or a wall that has been painted before.
How long should new render cure before painting?
New cement render generally needs about four weeks to cure before painting. Porter’s Paints’ technical specification for its Boncote cement paint lists 28 days for cement render and 14 days for a bagged brick surface.
Curing times vary with the product and the weather, so always follow the render maker’s and paint maker’s data sheets. Some paint makers offer sealers designed for fresh render that shorten the wait, which can help when a renovation is on a tight timeline.
How do you find hairline cracks in render?
The simplest test is to wet the wall. Porter’s specification describes hosing down the render and watching it dry, because hairline cracks show up clearly as the surface dries around them.
It is worth doing this before painting, because fine cracks are much easier to see on bare render than after a coat of paint has gone on.
Do hairline cracks need fixing before painting?
Fine, non-structural hairline cracks can usually be treated as part of the paint job. Porter’s Paints makes a cement crack sealer designed to bridge non-structural hairline cracks up to 0.3 mm wide, and flexible exterior systems can also cover fine cracking.
Wider cracks, cracks that keep growing, or render that sounds hollow when tapped are different. They can point to movement or moisture behind the render, and they need to be repaired, or assessed by a builder or renderer, before painting.
What is efflorescence?
Efflorescence is the white, powdery salt deposit that can appear on render, brick and concrete. Water moving through the wall carries salts from the cement to the surface, where they are left behind as the water evaporates.
Paint applied over efflorescence will not bond properly, so the salts need to be removed first, and the source of moisture dealt with if the deposits keep coming back.
Will paint fix a damp rendered wall?
No. Paint can protect sound render from the weather, but it cannot stop moisture that is already getting in behind it. Rising damp, leaking gutters, poor drainage at the base of a wall or cracked render can all let water in, and paint over a damp wall tends to bubble, peel or grow mould.
If a rendered wall stays damp, stains repeatedly or keeps producing efflorescence, find and fix the source of the water first. Once the wall has dried out, it can be prepared and painted properly.
What paint should you use on render?
Use an exterior paint system designed for masonry. Dulux notes that fresh cement render is highly alkaline, and that conventional low build paint coatings are not recommended for cement render. A suitable sealer or primer goes on first where the render is new or porous, followed by the topcoats.
On exteriors, we use the Haymes Solarshield system, with the primer and preparation chosen to suit the condition of the render. You can read more on our exterior repainting page.
What is the difference between render and bagged brick?
Cement render is a layer of cement-based mix applied over masonry and finished smooth or textured. Bagging is a thinner coat rubbed into brickwork, so the shape of the bricks still shows through.
Both are painted in a similar way, but they cure at different rates. Porter’s specification allows 28 days for new cement render and 14 days for a newly bagged surface before painting. For 1950s brick and rendered homes, see our Balwyn North painters page.
What does a render repaint involve?
Repainting previously painted render usually follows these steps:
- Wash the walls to remove dirt, chalky residue and any mould.
- Scrape back loose or flaking paint.
- Wet the wall and check for hairline cracks as it dries.
- Fill or seal fine cracks, and have wider cracks repaired first.
- Remove any efflorescence and prime bare patches.
- Apply two topcoats of an exterior masonry system.
Can you paint over previously painted render?
Yes. Previously painted render is washed, any loose or flaking paint is scraped back, cracks are filled, and bare patches are primed before the new topcoats.
Render that has never been painted is a bigger decision, because once it is painted it will need repainting over time. In a heritage overlay, painting a surface that has never been painted needs a council permit. Our Malvern painters page explains how that rule works in Stonnington.
Frequently asked questions
Can I paint render that has never been painted?
Yes, but think it through first, because painted render needs repainting over time. If the property is in a heritage overlay, painting a previously unpainted surface needs a council permit.
Why is my rendered wall turning white and powdery?
It is usually efflorescence, where salts carried by moisture are left on the surface as it dries. The salts need removing, and the moisture source fixing, before the wall is painted.
Does new render need a sealer before painting?
Usually, yes. New or porous render is normally sealed or primed before the topcoats go on. Follow the paint maker’s data sheet for the right product and timing.
If you have rendered walls to paint, Steve can look at the render and talk you through the options. Call 0419 581 575 for a free written quote. For timber homes, see our guide to painting weatherboards.