Prep Guide: Sealing and Priming Brick and Masonry

Brick fails paint in one of two ways: it drinks the water out of it, or it pushes salts through it. Sealing is not a single product decision — it is a moisture test, a repoint, and a primer choice, in that order.

Substrate

Brick

Full cure

28 days

Touch dry

6 hours

Steps

6

Test for moisture before you buy anything

Tape a 300 mm square of clear plastic flat against the wall and leave it 24 hours. Then read the result:

What you see	What it means	Action
Condensation on the inside of the plastic	Moisture is coming through the wall from outside or below	Do not paint. Find the source — a failed gutter, a bridged damp course, raised ground level
Wall face is damp, plastic is dry	Residual construction moisture or a cold wall collecting condensation	Ventilate and wait; a cold wall may need insulation, not paint
Both dry	The wall is ready to seal	Continue with the sequence below

Painting a damp wall traps the moisture behind the film. It will blow within one heating season, and the failure always happens in the largest visible panel.

Repointing, and why lime not cement

If the mortar is crumbling, cut it out to a depth of about twice its width and repoint with a lime mortar. Cement mortar is harder and less flexible than the brick around it. When the wall moves with temperature and moisture, the cement does not, so the stress is transferred into the brick face — which is how brick spalls. Lime mortar is sacrificial: it fails first and is cheap to replace.

Point the joint slightly recessed rather than flush, and tool it with a rounded profile so water runs off instead of sitting on the ledge.

Primer choice

Three products get called "brick sealer" and they do different jobs:

• Stabilising primer: soaks in and binds dusty, friable surfaces. Use it where a brush raises a cloud of dust.
• Alkali-resisting primer: for new mortar and render, which is strongly alkaline for the first few months and will burn through a standard primer.
• Water repellent: for exterior brick that will stay unpainted. Never use it on a wall you intend to paint — paint does not adhere to a water repellent film.

For interior brick that will be painted, the stabilising primer is usually the right one. Apply it with a masonry brush, work it into the joints, and leave it 4 hours.

The dampening technique

Spray the wall with clean water and wait until the surface sheen disappears and the brick looks matt again — five to ten minutes in a warm room. Then paint. This is the single technique that makes the difference between a brick wall that lasts and one that fails: it stops the substrate from pulling water out of the paint before the film can form.

Check the room humidity while you work. Above 70% RH the primer and the first coat will both take far longer to dry, and the risk of a soft film goes up sharply.

What matters here

• Test for moisture with a plastic square for 24 hours before buying any sealer.
• Repoint with lime mortar — cement spalls the brick it surrounds.
• Stabilising primer for dusty brick, alkali-resisting for new mortar.
• Damp the wall before every coat and paint when it goes matt again.

Elsewhere on this wall

• Prep Guide: First Coat on New Plasterboard
• Prep Guide: Stripping Old Paint Before Recoating