What is efflorescence in masonry?

Efflorescence is the term most commonly used to describe the deposit of crusty white mineral salts that appear on a masonry surface (concrete, render, brick or mortar) that have leached out from within the substrate when moisture migrates through it.

Any efflorescence on the surface must be removed prior to painting as it is regarded as a poor and friable base that prevents paints and coating systems from adhering effectively.

Paint systems adhering normally to the surface of the substrate can also be forced off (delaminated), when the pressure caused by the growth of salt crystals builds up beneath the paint film, resulting in its gradual but irreversible destruction.

Since the mineral salt crystals are not fluid, the pressure is therefore not uniform, so the coating does not form smooth rounded blisters.

The paint film may stretch to form the outline of the growing crystals or the paint film may just rupture, crack, flake or peel.