Painting concrete block: sealing, breathability and what goes wrong

Concrete block takes paint well once it is fully cured, clean and dry, and the coating you choose has to let water vapour back out. Nearly every block wall paint failure is moisture pushing outward from behind a film that is too tight, not a fault in the paint.

That one fact decides most of what follows: what you use, when you can start, and whether a particular wall should be painted at all.

How long does new block have to cure?

Longer than most people want to wait, and the clock starts again if a solid plaster coat goes over the top of the block. Paint and sealer manufacturers publish a minimum cure time for new cementitious surfaces on the data sheet for the product being used, so take the figure from there rather than from the builder's programme, and count it from the day the plaster went on rather than from the day the blocks were laid. Fresh cementitious surfaces are strongly alkaline and still full of free moisture, and coating into that gets you three problems at once: patchy colour where the alkali attacks the pigment, salts driven to the surface, and weak adhesion because the film never got a dry substrate to bond to.

Deep and strongly saturated colours suffer worst, because there is more pigment for the alkali to burn. If a builder is pushing for paint on a wall that is only a few weeks old, the answer is to wait, or to use an alkali-resistant sealer specified for early coating and accept that it is a compromise.

Two checks are worth doing before anyone opens a tin: a moisture meter reading on the block itself, and a pH test with an indicator strip on a damp patch of the surface. Neither is difficult, and both cost less than repainting.

What is the white powder on the wall?

Efflorescence. Water moving through the masonry dissolves salts, carries them to the surface and evaporates, leaving the salt behind as a white crystalline bloom. It is not mould, and hosing it off achieves nothing useful, because the water dissolves the salt and carries it straight back into the wall.

Brush it off dry with a stiff brush, then wash the wall and let it dry fully. If it comes back, you have an active water path and no paint will fix it. Find where the water is getting in: a failed cap or coping on top of the wall, missing or blocked drainage behind a retaining wall, a downpipe discharging against the base, or ground built up above the damp-proof course. Painting over recurring efflorescence traps the salt under the film and the film comes off.

What does breathable actually mean here?

It means the coating lets water vapour pass through while still shedding liquid water off the face. Masonry absorbs and releases moisture constantly, which is what it is meant to do. A vapour-permeable masonry acrylic works with that. A tight, low-permeability film traps vapour behind it, and once the vapour pressure exceeds the adhesion of the paint, the paint blisters off in coin-sized discs with damp grey block behind them.

The wall where this is close to guaranteed is one with soil against the back of it: a retaining wall, or the buried side of a basement or garage wall. Water enters the buried face and can only leave through the face you can see. Coat that face with something tight and it will fail, however good the product is and however careful the preparation was. These walls are everywhere on Wellington's steep sections, and they are the most common block-painting disappointment we get called to.

What should go on which wall?

Wall	What usually goes wrong	What it needs
New block, not yet fully cured	Alkali burn, patchy colour, efflorescence	Wait, or use an alkali-resistant sealer specified for early coating
Open-texture or honeycombed block	Pinholes, poor coverage, far more paint than the area suggests	Block filler or high-build masonry sealer, then two finish coats
Retaining wall with soil behind it	Blistering driven from the back	Fix the drainage first, then a vapour-permeable coating only
Basement or garage wall cut into a hill	Damp patches and blisters near the base	Deal with the ground water, permeable coating, expect a shorter recoat cycle
Exposed coastal block	Salt deposits, early chalking	Regular fresh water washing and a durable exterior masonry system
Sound painted block, weathered but intact	Chalking and fading	Wash, spot prime bare areas, two coats over the top
Interior garage or basement block	Dust, efflorescence, condensation	Seal, then a washable coating, and ventilate while it dries

How much more paint does the texture drink?

A lot. The coverage figure on a tin assumes a smooth sealed surface, and rough block has far more actual surface area than its measured square metres suggest. On open-texture block, expect the first coat to use a great deal more than the same area of smooth wall would take, and buy paint on that basis rather than on the tin's figure. That is why a block filler or high-build sealer usually pays for itself: it fills the pinholes so the finish coats can do their own job instead of acting as filler.

If a quote for a block wall looks high next to a quote for a plastered wall of the same size, this is why, and it is legitimate.

Do the cracks need fixing first?

It depends entirely on the kind of crack, and this is worth getting right in a city that shakes.

Fine shrinkage cracking in a plaster coat over block, or hairline movement along a mortar joint, is a filler and flexible sealant job before painting. It is normal, and a high-build or elastomeric coating will bridge small ongoing movement afterwards.

Cracking that steps diagonally through both the blocks and the mortar joints, that is wider at one end than the other, or that has one side sitting proud of the other, is structural. That is an engineer's assessment, not a painter's. Filling it and painting over it hides useful information and buys you nothing. If you are unsure, mark the ends of the crack with a pencil and the date, photograph it, and look again after a few months and again after a winter. A crack that has not moved is a different problem from one that has.

Should you paint block at all?

Not always. Sound fair-faced block that is not staining and not shedding does not need a coating, and painting it commits you to repainting it every time it weathers. The good reasons to coat block are appearance, evening out patch repairs and mismatched mortar, and shedding wind-driven rain off a badly exposed face. Sealing it is not a reason on its own, and on a wall with moisture behind it, sealing is exactly the wrong instinct.

Commercial and industrial block is a slightly different calculation, because a coated wall is easier to clean and to keep clear of graffiti. That sits under commercial painting and is usually specified around durability rather than looks.

What to ask before anyone starts

Three questions. What is the moisture content of the wall right now. What coating is being specified, and is it vapour permeable. What happens at the base of the wall, where it meets the ground. Anyone who has painted block on a Wellington hillside will have an answer to all three, and the answers tell you more about the job than the price does. Block is often coated as part of a wider exterior repaint, and it should be priced as its own item, because it behaves nothing like weatherboard.

If you have a block wall that keeps blistering or blooming white, the paint is telling you something about water. Send us a photo and we will tell you what. Get in touch.