Efflorescence or formation of white powdery deposit on walls after painting is caused due to salts present in the building material like brick and mortar, which surface later on.
Prevention[]
Give a long time gap between plastering and painting (about 6 months including one monsoon) Use paint with a porous film like emulsions and distempers.
Cure[]
Wipe off the deposited salts completely from the surface and ensure no more deposition before recoating.