To replicate the popcorn texture requires a professional plasterer.
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.
If you have uninsulated exterior walls, you likely haven't fully sealed your home’s “thermal envelope.”
Adding insulation in plaster walls with a brick exterior can be a challenge, especially if there are large cracks in the plaster.
Chances are you'll need a professional to address any large cracks in plaster walls before and after the job.