Cause
Efflorescence can originate from any cementitious building products such as brick, concrete blocks, sand: cement rendering and plaster. As moisture or moisture vapour passes through the materials to the surface it will bring any soluble salt deposits within the substrate to the surface. Initially the deposits appear fluffy and can be brushed off. If not removed this will crystalize, build up and damage the surface finish. The salts can remain dormant for many years and efflorescence can appear if the building substrate is affected by water ingress due to water leaks, flooding or a faulty damp proof lining.
Remedy
Decorating should be left until salts cease to appear by constant dry brushing. If painting is necessary then it is best to apply water based coatings of a high pigment content which allow moisture vapour to pass through the paint film more readily than others, allowing further salts to come to the surface without causing any major problems.
Some suppliers recommend treating the area with an alkali resisting primer sealer and while this will reduce any moisture vapour coming to the surface it is possible that any underlying salts could re-appear from the unsealed surrounding areas.