How to Patch a Damaged Panel of Acoustic Plaster
Acoustic plaster is a porous finish, and a patch fails visibly when it is repaired like a solid one — the new area reads as a smooth bruise under raking light. The repair is five steps, and four of them happen before any new material goes on the wall.
- Time needed
- About six hours, over two days
- Number of steps
- 5 steps
What You Need
Tools
- A scoring knife with a fresh blade
- A stainless finishing trowel, worn rather than new
- A sponge float for texture matching
- A raking lamp on a stand
Supplies
- Acoustic plaster base coat, matched to the original system
- Acoustic finish coat in the original grade
- Mineral primer for the cut-back substrate
01Find the real edge of the damage under raking light
Set a lamp almost flat against the wall and mark where the surface stops being sound. Damage in a porous finish runs further than it looks under ambient light, and a patch cut to the visible edge will lift at its boundary within a season.
- Rake the light from at least two directions
- Mark in pencil, never in tape — tape lifts the surface
- Expect the sound edge 50 to 100 mm beyond the visible one
02Score the boundary before cutting anything back
Score a clean line at the marked edge, through the finish and into the base coat. Cutting back without scoring first tears the surrounding material, which enlarges the repair every time and turns a patch into a wall.
- One continuous pass, fresh blade, moderate pressure
- Score into the base coat, not through to the substrate
- Irregular boundaries hide better than rectangles
03Cut back and feather the edge
Remove everything inside the score line and then feather the last twenty millimetres so the new material has a taper to key into rather than a step. A square edge telegraphs through the patch as a visible outline no matter how well the texture is matched.
- Feather the last 20 mm to nothing
- Vacuum the cut-back area rather than brushing it
- Prime the exposed substrate and let it go off fully
04Build the base coat in two passes, not one
Fill to just below the surrounding level in the first pass and let it cure before the second. A single thick pass shrinks as it dries and pulls the feathered edge with it, which reopens the boundary the previous step just closed.
- First pass 2 mm below the surrounding surface
- Full cure between passes — overnight, not an hour
- Second pass flush, worked out past the feather
05Match the texture wet, and check it dry
The finish coat is matched with the sponge float while it is still open, working from the centre outward so the texture dies into the existing surface rather than stopping at it. Then leave it, and check under the raking lamp once it has dried — the sheen only settles at full cure, and a match judged wet is usually a shade flatter than it looked.
- Work centre-outward, never edge-inward
- Match texture wet; judge sheen only when fully dry
- Re-check under the same raking light used in step one