Cavity walls are one of the most misunderstood surfaces in Australian residential construction. They consist of two parallel masonry leaves separated by an air gap, typically 50–75 mm wide, designed to keep water from migrating inward. That gap is the key to the whole system. Paint a cavity wall incorrectly and you can seal moisture pathways, cause efflorescence to bloom through the surface, or accelerate corrosion on the wall ties hidden inside. Getting the approach right starts with understanding what's actually going on behind the face course.
Why cavity walls behave differently under paint
Solid brick walls absorb and release moisture in a fairly predictable cycle. Cavity walls don't. The outer leaf is exposed to weather and wets up regularly. The inner leaf stays drier. Between them, the cavity is supposed to drain any water that penetrates the outer course down and out through weep holes at the base. Block those weep holes, or apply a non-breathable paint film to the outer face, and the drainage system fails. Water builds up inside the cavity, finds the path of least resistance, and pushes outward as the wall dries, taking your new paint with it.
In Melbourne specifically, where temperature swings between seasons are significant, this wetting-and-drying cycle is aggressive. An exterior film that can't move moisture vapour outward will blister. It won't happen immediately, but within one or two winters the failure is visible. Efflorescence, white crystalline salt deposits, appears where the moisture is moving fastest.
Assessing the wall before you open a tin
Don't skip the inspection phase. Before any prep work, check the base of the outer leaf for weep holes. These are intentional gaps in the perpendicular mortar joints at the lowest course, usually every 900 mm or so. If they've been pointed over by a previous owner, open them carefully with a plugging chisel. A blocked weep hole is often the root cause of a failed paint job on the wall above it.
Next, tap the face of the wall systematically. A hollow sound can indicate that a previous paint layer has lost adhesion or that the outer render coat is delaminating from the masonry beneath. Any hollow sections need to be cut back and re-rendered before painting. Painting over delaminated render just delays the failure by a season.
Check the mortar joints as well. Cavity walls in homes built before 1980 often have lime-based mortar that weathers and recedes, leaving open joints that water tracks through directly. Rake and repoint any joints showing a recession of more than 5 mm before priming.
Product selection: breathability is non-negotiable
The most important property of any paint applied to a cavity wall exterior is vapour permeability. You need a product that allows moisture vapour to pass outward while resisting liquid water ingress from outside. Elastomeric masonry paints and specialist breathable acrylics both meet this requirement. Standard interior acrylics and low-cost exterior paints generally don't.
Look for products with a water vapour transmission rating (often listed as Sd value) below 0.1 m. Dulux Weathershield and Taubmans All Weather are two locally available products formulated for Australian masonry exteriors with appropriate breathability. Either is a sound choice over a properly primed cavity wall surface.
Avoid solvent-based paints on the outer leaf of a cavity wall unless the manufacturer explicitly lists masonry vapour management as a feature. Oil-based coatings can form a film that traps moisture against the face of the brick and guarantees efflorescence within two years.
Priming: the step most DIYers underweight
A dedicated masonry sealer or alkali-resistant primer is mandatory on any cavity wall that hasn't been painted before, or that has been stripped back to bare masonry. Fresh mortar and brick are alkaline, and that alkalinity will saponify many standard primers, causing them to break down and the paint film to lift. A purpose-made alkali-resistant primer neutralises the surface and creates a stable base.
Apply the primer liberally. Masonry is porous and will absorb the first coat unevenly. A second coat of primer over especially thirsty sections is cheaper than a second topcoat later. Let the primer cure fully, according to the product datasheet, before applying finish coats. In cooler Melbourne autumn weather, this can take 24–36 hours rather than the 4 hours listed for summer conditions.
One place where primer selection really matters is around window and door reveals. These junctions are where the two leaves of a cavity wall often connect through a closer brick, and moisture tends to concentrate there. For guidance on handling these junctions when painting, the approach described for painting a bay window reveal carries over well to cavity wall construction.
Application technique for the outer leaf
A thick-nap roller (18–20 mm) is the right tool for most cavity wall brick faces. It forces paint into the textured surface without bridging over the voids between mortar and brick. Bridging is how pinholes form, and pinholes are entry points for water under the film.
Work in sections of roughly 2 square metres. Roll the first coat in one direction, then cross-roll immediately before it starts to set. This technique presses the paint into the surface rather than just coating the high points. Apply two full finish coats with a minimum 4-hour recoat window between them in warm conditions, longer in cool or humid weather.
Brush application works well for mortar joints, but avoid overloading the brush. Thick paint sitting in a mortar joint can block the micro-drainage pathways that are part of how the outer leaf manages water. Light, even passes are better than heavy fills.
Timing the job around Melbourne's weather
Cavity wall painting is particularly sensitive to ambient conditions. Applying paint in cool, damp weather slows the cure dramatically and can leave the film tacky long enough to collect airborne dust. Applying in very hot conditions causes the solvent to flash off too quickly before the paint has bonded properly. Both extremes damage the finish.
The ideal window is a dry day between 10°C and 28°C with low humidity and no rain forecast for at least 24 hours. Melbourne's spring (September to November) typically offers the most reliable run of suitable days, and autumn (March to May) is the second-best option. For a broader look at how Melbourne's climate affects paint timing, the advice in the article on choosing exterior paint for Melbourne's climate applies directly to cavity wall projects.
After the job: maintaining the finish
A breathable masonry paint on a properly prepared cavity wall should hold for 8 to 10 years before it needs recoating. Between those repaints, wash the surface annually with a low-pressure garden hose and a soft brush to clear biological growth and salt deposits before they build up enough to attack the film. Don't use a high-pressure washer on older mortar joints. The force can dislodge repointed sections and open new water pathways into the cavity.
If efflorescence reappears within the first 12 months, it signals that a moisture source is still active, either a blocked weep hole, a failing mortar joint above, or a compromised damp-proof course. Address the source before re-painting. Painting over efflorescence without fixing its cause produces the same result every time.
Apex Painting handles cavity wall exterior repaints across Melbourne, selecting breathable coatings matched to each substrate and inspecting weep holes and mortar before any preparation begins. Correct diagnosis before the first brush stroke is what separates a lasting result from a paint job that's back to blistering by winter.