Quick answers
The plaster sounds hollow when I tap it. What does that mean?
That the plaster has come away from the wall behind it, even if it doesn't show yet. It isn't filled over: it is removed back to a sound base and redone.
Sound plaster is bonded tightly to the brick or concrete, and when you tap it lightly with your knuckles or the handle of a tool, it sounds solid. When it has come away, there is a gap between the plaster and the wall, and the sound is dull. Often nothing shows yet, or there are only fine cracks and blisters in the paint.
The most common cause is damp, which over the years ruins the plaster's grip. Other causes are age in old houses, vibration and, on ceilings, rust on the steel bars pushing the material outwards.
Hollow plaster can't be fixed with filler and paint. Everything that has come loose is removed until a sound base is reached, even if the piece turns out bigger than it looked. On a courtyard wall, the plaster had blistered and fallen off in large pieces. Everything loose was removed, the gaps were filled with new plaster, mesh went over the patches, and a thin coat of plaster joined the whole surface together before painting.
Keep in mind: on a ceiling, hollow plaster can fall. If it sounds hollow over a large area or is sagging, keep the space underneath clear until someone has looked at it. And if the cause is damp, that has to be dealt with first, or the new plaster will come away too.
Jobs that show it
Courtyard Wall: Plaster Repairs and Fresh Paint
An outside wall with blistered and fallen plaster was patched, reinforced with mesh, given a thin finishing coat and painted white.
See the job step by step
Ceiling Crack Repaired and Bedroom Repainted
A crack in the ceiling had reached the steel reinforcement. It was repaired and the room was painted again.
See the job step by step
Repairing a Peeling Wall by a Door
Paint that had lifted in large patches was scraped off, and the wall was filled and painted again.
See the job step by step