White patches of salts that penetrate from the plaster or bricks impacted by water.
Causes:
The surface has not been sufficiently prepared – the salt layer has not been removed from the surface prior to painting;
Penetration of humidity from the external walls – constructive defect.
Solution:
If excessive humidity is the cause, prevent its occurrence: repair the roof, clean the gutters and drain pipes, seal the cracks in the façade, consider installing vents or fans, particularly in the kitchen, bathroom and laundry room. Rub down the salt layer with a wire brush and wash with water, using a pressure cleaner. Then carefully rinse the surface. When the surface is dry, repaint it with a high-quality outdoor paint.