Concrete block walls, also called besser brick or CMU walls, turn up constantly in Melbourne: boundary fences, garage interiors, commercial outbuildings and the lower courses of many older brick homes. They're dense, heavy and look permanent. The paint on them, though, is another story. Besser brick is highly porous and carries residual alkalinity that eats through standard primers. Get the prep wrong and the finish flakes within a season. Get it right and a painted concrete block wall holds up for a decade without major attention.
Why concrete block is different from standard brick
Standard clay bricks and besser blocks are both masonry, but they behave differently under paint. Besser block is cast from Portland cement, sand and aggregate. That cement content means the surface stays alkaline, sometimes for years after the block is laid. Alkalinity saponifies oil-based coatings, turning them soapy and causing them to delaminate. Even some water-based products fail if the pH is high enough.
Porosity is the second problem. The coarse, open texture of a concrete block face absorbs an extraordinary amount of product. A litre of paint that covers 16 square metres on a smooth wall might cover 8 square metres or less on raw besser. Underestimating this is the most expensive mistake first-timers make, because they run short mid-job and apply thin coats trying to stretch coverage.
Moisture movement is the third factor. Concrete block wicks groundwater from below and draws condensation in from either face depending on temperature. Any coating that doesn't allow vapour to pass through will trap that moisture and eventually blister.
Testing before you prime
Two quick tests save a lot of grief. First, sprinkle a small amount of water on the surface. If it beads, there's a sealer or old paint film present. If it soaks in within seconds, the block is raw and porous. Second, buy a cheap pH testing kit from a hardware store and apply it to the damp surface. A reading above 10 means the alkalinity is high enough to damage unsuitable primers. At that level, an alkali-resistant primer is non-negotiable, not optional.
Also check for efflorescence. Those white, powdery deposits on the face of the block are mineral salts pushed to the surface by moisture movement. They must come off before any coating goes on. Scrub them with a stiff brush and a diluted masonry cleaner, then rinse thoroughly and allow at least 48 hours to dry.
Filling, patching and crack repair
Besser block walls frequently have surface voids from the casting process, honeycombing at corners, and mortar joints that are slightly recessed or cracked. Each of these needs attention before priming.
Use a cementitious patching compound rather than standard filler for any void deeper than 5 mm. Standard interior fillers aren't alkaline-resistant and will fail at the interface. For hairline cracks in mortar joints, rake out any loose material and repoint with a matching mortar mix. Let all patches cure for at least 72 hours before continuing.
If the entire wall face is rough and the owner wants a smoother finish, a skim coat of render is worth considering. This adds cost and time but produces a surface that paints far more evenly and resists moisture better than raw block. For industrial or utility spaces where appearance is secondary, painting the raw block face directly is fine, provided the primer is right.
Choosing and applying the right primer
This is where most DIY besser brick paint jobs go wrong. The primer must do three things at once: seal the porous surface against excessive absorption, neutralise or tolerate residual alkalinity, and bond firmly enough to anchor the topcoat under moisture cycling.
A penetrating alkali-resistant masonry primer is the correct choice for raw or recently laid block. Products specifically labelled for masonry or concrete surfaces and rated for high-pH substrates are what to look for. Apply the primer generously and work it into the surface texture with a thick-nap roller (at least 18 mm nap). A 20 mm nap roller is better on coarse block faces. Brush the mortar joints first, then roll the flat faces.
One coat of primer is rarely enough on raw besser. Plan for two coats and check coverage before committing to a topcoat. The surface should look uniformly sealed with no dry, powdery patches showing through.
Topcoat selection
For exterior concrete block walls exposed to Melbourne's weather, an acrylic masonry paint with a flat or low-sheen finish is the standard choice. Flat finishes hide texture variation and surface imperfections better than semi-gloss. They're also more breathable, which matters on a surface that moves moisture. If the wall faces west or gets direct afternoon sun, choose a product rated for UV resistance, as cheaper masonry paints fade noticeably within two summers.
Interior besser block walls, such as garage interiors, storage rooms or lower-ground-level utility spaces, can use a standard interior acrylic if the pH test was acceptable and the primer was applied correctly. A low-sheen or eggshell finish is easier to clean than flat and still breathable enough for most interior masonry contexts. For high-moisture interior spaces like subfloor areas, stick with an exterior-rated masonry product even indoors. The extra durability is worth it.
Two topcoats are standard on besser block. Three is better if the surface is rough and the first two coats have been absorbed unevenly. Don't stretch coats thin trying to save product. A thin coat on porous masonry loses adhesion faster than a properly applied full-thickness coat.
Application technique on a coarse surface
Rolling a concrete block wall is physically harder than rolling plaster. The coarse aggregate grabs the roller and resists even movement. Use a sturdy roller frame, not a cheap plastic one, because the lateral forces on a rough masonry surface will flex a flimsy frame and produce uneven pressure.
Work in 1-metre sections. Load the roller fully, apply with vertical passes, then cross-roll horizontally to work the product into the texture. Don't race. Thin spots on the flat face of each block are the most common source of early paint failure on besser, and they're caused by moving too fast.
Brush the mortar joints before each roller pass. The joints are recessed and the roller skips across them rather than pressing into them. A 50 mm brush, stippled rather than stroked into each joint, fills them properly.
Dealing with old paint on besser block
If the wall has been painted before, test adhesion by applying a strip of masking tape firmly across the surface, then pulling it off quickly. If paint lifts with the tape, the existing film is poorly bonded and needs to come off before any new coating goes on. Pressure washing at 2500–3000 psi removes loose, chalky or flaking paint effectively. Allow 48 hours to dry, then sand any remaining high edges flat before priming.
For a wall with a sound existing paint film, clean it with sugar soap, rinse well, allow to dry, then apply a bonding primer over the existing coat before topcoating. Painting directly over an existing topcoat without a bonding step is one of the most common sources of inter-coat delamination on masonry. You can read more about this kind of prep failure in the broader context of how to paint a brick fireplace surround without flaking, where similar alkalinity and adhesion dynamics apply.
How Melbourne's climate affects the job
Timing matters on masonry more than on most surfaces. The concrete block face needs to be dry before priming, which means waiting at least 48 hours after rain and checking that the substrate temperature is above 10°C. Cold block below that threshold slows cure times significantly and can leave a primer film that never reaches full hardness before the topcoat goes over it.
Summer heat is the opposite problem. Besser block in direct sun can reach 50°C or above, and applying primer or paint to a surface that hot causes it to dry before it can penetrate. Paint in the early morning on hot days, working in shade wherever possible. For a more detailed look at how Melbourne's climate interacts with exterior coating choices, how to choose exterior paint that handles Melbourne's climate covers the seasonal considerations in depth.
Finishing and long-term care
Once the final topcoat has cured (allow a full 7 days for maximum hardness on masonry), inspect the wall in raking light. Any skipped spots or thin sections will be obvious. Touch these up with a brush rather than a roller to avoid creating visible sheen differences across the face.
Clean the wall annually with a mild detergent and soft brush to remove mould, algae and general grime. Melbourne's combination of humidity and tree cover makes biological growth on masonry surfaces a realistic ongoing problem. A quality masonry paint with a built-in fungicide additive reduces regrowth between cleans, but it doesn't eliminate the need for annual maintenance entirely. Inspect the mortar joints every 3 years and repoint any that have cracked or receded, because open joints are the primary entry point for moisture that eventually pushes paint off the face from behind.
Done properly, a painted concrete block wall in Melbourne should hold its finish for 8 to 10 years before a full repaint is needed. The preparation work is the hard part. The painting itself, once the surface is properly sealed, is straightforward.