Do we need to repair plaster before repainting?by Nancy Minter on 06/28/16
It's always a good idea to repair damaged plaster before painting.
For paint to adhere properly and look good, surface condition and preparation are very important. A damaged surface should be repaired properly, then primed and painted.
If plaster is water damaged, and you just paint over the damage, the paint will fail. It might chip, crack or eventually it could bubble. Even worse, because plaster is strong but brittle, a damaged section could fall and even hurt someone.
Determine the cause of the damage
Before you begin to repair or paint, be certain of the cause of damage. Fix any leaks or broken pipes and remove old insulation. This is a job best left to a pro.