How do I seal plasterboard where the paper face has torn off?

Plasterboard is a gypsum core with paper faces, and the paper is not decorative — it is the strong part of the board and it is the surface everything else is designed to bond to. When it tears off, usually from wallpaper stripping or tile removal, what is left is soft, dusty, absorbent gypsum. Anything water-based applied to it soaks in, swells the fibres and dries rough.

What you'll need

• Drywall sealer or a stabilising primer
• A brush or roller
• Fine surface filler or jointing compound
• Sanding block
• A scraper

Step by step

1. Remove anything loose first.Scrape away torn flaps of paper and any adhesive residue with a scraper held flat. Leaving a loose edge under the sealer means it lifts later and takes the repair with it.
2. Let the wall dry completely.If the damage came from steaming or washing, the board core will have taken up water. Sealing damp board traps the moisture and the sealer will not bond.
3. Choose a sealer designed for the job.A drywall sealer, a stabilising solution, or a specialist plasterboard primer. Ordinary emulsion is too thin and simply soaks in. Neat PVA is a common suggestion and a poor one — it forms a plastic film that softens when wet and gives paint a surface it cannot key to.
4. Apply it generously and let it soak in.Well beyond the damaged area and onto the sound paper around it, so there is no hard boundary. A second coat where the first disappeared instantly.
5. Let it dry fully.The surface should feel bound and slightly harder, and it should not shed fibres when rubbed with a fingertip. If it still does, apply more.
6. Now fill.Fine surface filler or jointing compound across the area with a broad knife, in thin coats, feathered well out. Filling before sealing is what produces a sunken, rough patch.
7. Sand lightly and prime again.The filler is itself absorbent, so prime over it before the topcoat or it will flash as a dull patch through the paint.
8. Consider skimming the whole wall if the damage is extensive.Where a wall has lost its paper face over large areas — a common result of removing tiles — sealing and skimming the whole wall is a better outcome than dozens of patches.

Tips

• The test for whether the sealer has done its job: rub the dry surface firmly with a fingertip. Nothing should come away and no fuzz should raise.
• If the board is soft as well as torn — you can push a fingernail into the core — it has been wet, and it needs replacing rather than sealing.
• This same sealing step applies before tiling onto damaged board, and before skimming, not just before painting. Any wet material applied to bare gypsum core has the same problem.

Common mistakes

• Painting straight onto the bare core — Emulsion soaks in, swells the loose fibre, and dries as a rough, patchy area that gets worse with each coat.
• Using neat PVA as the sealer — It leaves a glossy plastic film that later paint cannot key into properly, and it re-softens when it gets wet. A purpose-made drywall sealer does the opposite: it penetrates and binds.
• Filling before sealing — The filler's water is drawn straight out into the core, so it goes off before it is worked, sits proud and dusty, and does not bond.
• Sealing only the damaged patch — The boundary between sealed and unsealed surface shows as a line of different absorbency under paint. Take it well out.

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