A property manager walked me down a ground-floor corridor in a mid-rise last month and pointed at a wall her painters had finished three weeks earlier. Two coats, good product, clean cut lines — and it still looked mottled, like the wall was drying in patches. She wanted to know whether she had been sold cheap paint. She hadn’t. The wall was concrete block, and nobody had filled it before painting. This post is for property managers and boards looking after buildings with block walls — stairwells, corridors, garages, mechanical rooms, amenity rooms — who keep getting a finish that never looks quite right.
1. Concrete block drinks paint, and it does not drink evenly
Concrete block is porous. Not a little porous — the face of a standard block is full of open pinholes and voids you can see if you get close with a light. Mortar joints are denser than the block itself, and even the blocks within one wall vary depending on where they sat in the pallet.
When you roll paint onto that, the wall absorbs it at different rates across a single square metre. The thirsty spots pull the binder down into the block and lose their sheen; the tighter spots hold the film on the surface and keep it. Stand back and it reads as blotchy, even though the applicator did nothing wrong and the film thickness is close to identical everywhere.
Adding a third coat of finish paint does not fix it. It costs the building another coat and the wall usually still flashes, because you are still feeding an unsealed substrate. I have watched boards pay for a fourth coat chasing this.
What to look at: find a block wall in your building and look at it with light skimming across the surface rather than straight on. If the block faces read duller than the mortar joints, the wall was painted without being filled.
2. Block filler is not the same thing as primer
This is the part that surprises most people. A primer is a thin sealing coat. Block filler is a heavy-bodied product, closer to a thick paste than a paint, and its job is to physically fill the pinholes and voids in the block face so the wall becomes one continuous, uniform surface.
It goes on far heavier than paint — usually worked into the block with a thick-nap roller, sometimes sprayed and back-rolled to force it into the voids. One properly applied coat of filler will use several times the material per square metre that a coat of finish paint does. That is not waste; that is the product doing its job.
Once the block is filled, the two finish coats sit on a uniform surface and cure the same way everywhere. The wall looks flat and even, it cleans properly, and the paint lasts because it is a real film instead of a thin skin stretched over open pores.
What to look at: pull the painting specification from your last block-wall project and search it for the words “block filler.” If it only says “primer and two coats,” that is where the money went missing.
3. The mortar joints will tell you what actually happened
If you want to diagnose a block wall without a lab, use the joints. On a properly filled wall the joints and the block faces look like one surface, and running a hand across it feels almost smooth. On an unfilled wall the joints stay visibly crisper and slightly glossier than the block, and the surface still feels gritty and open.
The second tell is washability. Block that was never filled is full of tiny craters that catch dirt and scuff marks, and cleaning staff cannot get them out — they scrub, the mark stays, and eventually somebody rubs through the paint. That is often the real complaint that reaches the board: not “the wall looks patchy” but “the corridor always looks dirty.” It is the same root cause, and it compounds with the sheen chosen for that wall.
What to look at: run your hand across a painted block wall in a high-traffic corridor. If it feels like fine sandpaper, no amount of repainting the same way will change how it cleans.
4. It matters most exactly where buildings cut the budget
Block walls tend to live in the places nobody tours: stairwells, service corridors, mechanical and electrical rooms, storage, garages, loading areas, and the back-of-house side of amenity rooms. Those are the first areas trimmed when a painting budget gets tight, and the last to get a proper specification.
They are also the areas that take the most abuse — carts, bikes, equipment, salt and damp through the winter. In a parking garage or a damp mechanical room the stakes go up again, because an unsealed block wall lets moisture move through the coating and push the paint off from behind. That is a coating-system question rather than a paint question, the same way exterior steel needs a system rather than a colour.
What to look at: walk your stairwell and your garage with the same eye you use on the lobby. Those are the walls where doing it properly once is dramatically cheaper than repainting every few years.
What to do if you’re seeing any of this
If a block wall in your building looks patchy after a fresh repaint, do not order another coat. Get someone to confirm whether the block was ever filled — it takes about two minutes with a light and a hand — because that determines whether you are looking at a touch-up or a proper reset of that wall.
If you are specifying work for this year, make sure the words “block filler” appear in the scope for every block surface, and make sure the quotes you compare all include it. A bid that skips it will always look cheaper and will always cost more over the cycle. Our commercial and institutional painting services cover this work across Ottawa, and if you want a rough sense of what a block-wall program costs before taking it to the board, our painting cost estimator is a reasonable starting point.
I assess block-wall and coating conditions across Ottawa — condo corporations, multifamily portfolios, schools and institutional buildings. If you want a look at yours, call me at 613.325.3011 or email yasir@heims.ca.
— Yasir, HEIMS Construction
HEIMS Construction is an Ottawa-based commercial and institutional painting and drywall contractor serving Ottawa and the surrounding region. We are fully licensed, carry WSIB coverage and $5M commercial general liability insurance, and are members of BOMA Ottawa.