How to Repair Blown Plaster
Blown plaster is plaster that has lost its bond with the wall behind it. It is still sitting there, but it is no longer attached. You can usually hear it before you see it - tap the wall and it sounds hollow instead of solid. Left alone it will crack, bulge, and eventually fall off. Filling or painting over the top does nothing because the plaster underneath is no longer connected to the substrate. It needs to come off and be rebuilt.
How to spot blown plaster
Tap the wall with your knuckles. Sound plaster gives a solid, dull thud. Blown plaster sounds hollow. You might also see hairline cracks running across the surface, slight bulging, or areas where the plaster feels soft or moves when you press on it. Sometimes it is hidden behind paint and the only clue is a crack pattern that does not follow the usual settlement lines. Work outward from any suspect areas and tap as you go. Blown plaster often extends further than the visible damage suggests. Mark up the full extent of the hollow sections before you start cutting out - if you stop short, the plaster around the edge of your repair may blow later.
Step 1: Prep - Cut Out and Stabilise
Remove all the blown plaster. Use a bolster and lump hammer or a multi-tool, and cut back until you reach plaster that is firmly bonded to the substrate. You want a clean, solid edge all the way around the repair. Brush out all dust, debris, and loose material from the exposed masonry. The substrate needs to be clean and sound. If the exposed masonry or the edges of the remaining plaster are porous, powdery, or crumbling, apply Wall Hardener to stabilise the surface. Rub your hand or a dark cloth across the area - if it comes away dusty, it needs treating. Apply with a brush or roller and let it dry for a minimum of 24 hours before filling.
Step 2: Fill - Rebuild the Surface
Check the pack for mixing ratio of the powder filler. Add water to the tub first, then the powder. Mix to a smooth, lump-free consistency. To check that the consistency is right, load filler onto your filling knife and hold the knife vertically on its side. If the filler stays on the blade, the consistency is correct.
Using a filling knife or trowel, apply the filler firmly onto the exposed masonry, pressing it in to get a solid bond. Work from the edges of the repair inward, building the filler up flush with the surrounding plaster. These fillers handle any depth in one coat, so there is no need to build up in layers. For ceiling repairs, work in smaller amounts and apply firm upward pressure. Let the filler key into the substrate before loading more on. Once the filler has set, sand back with medium-grit sandpaper (120 grit) to bring it level with the surrounding plaster.
The repair is now complete and ready for paint.
Step 3: Finish - Smooth for Decoration
At this point, the repair is done. For a premium, paint-ready finish, apply your chosen Toupret finish product over the repair and feather it out across the surrounding area so there is no visible edge where the repair starts and stops. Let the finish coat dry completely, then lightly sand. The repair is ready for painting or wallpapering. Toupret fillers will not flash or grin through paint, so there is no need to spot prime.