Seal bare plaster before wallpapering

Seal fresh or old plaster with a diluted sealer before wallpapering, so paste doesn't soak in unevenly and paper won't slip or tear later.

AB-01Check the plaster is readyFresh plaster must be fully dry before you seal or paper it, or moisture gets trapped and the paper will blow later. As a rough guide, allow about a week of drying per mm of skim thickness in a normally heated, ventilated room — often 4-6 weeks for a standard skim. Dry plaster looks pale and even; damp patches look darker and feel cool or slightly tacky. Open a window or use gentle background heat, but don't blast it with a heater or fan, which can dry the surface too fast and crack it.

AB-02Clear and protect the roomMove furniture out or into the middle of the room and cover it with a dust sheet. Lay dust sheets over the floor and tape the edges to the skirting so drips don't get underneath. Take down curtains, blinds and any pictures still on the wall.

2 each · Dust sheet

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1 roll · Masking tape

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AB-03Work out which sealer your wall needsFor this job, it doesn't matter whether the wall behind the plaster is solid brick or block, dot-and-dab, or plasterboard on timber or metal studs — you're only sealing the skim surface, not fixing into the frame, so the stud type makes no difference. What does matter is whether the plaster is a modern gypsum skim or an older lime-based lath and plaster wall. Fresh gypsum skim (on any of the wall types above) takes a diluted PVA sealer well. Old lath and plaster is more porous and can be fragile, and PVA can seal it too hard and trap moisture, so use a proper plaster sealer or size instead, and test a small patch first.

AB-04Remove dust and loose materialBrush the whole wall or ceiling down with a soft, dry decorator's brush, working top to bottom. On lath and plaster, be gentle — hard scrubbing can knock loose crumbling patches. Vacuum up the dust from the floor and skirting afterwards so it doesn't end up in your sealer.

1 each · Soft dusting brush

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AB-05Fill any small cracks or holesFill hairline cracks, nail holes or small dents with an easy-fill filler using a small filler knife. Leave it to dry fully, then sand it flush with fine sandpaper. Don't skip this — sealer and wallpaper will show up any lump or dip.

1 each · Easy-fill filler, small tub

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1 each · Filler knife

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1 sheet · Fine sandpaper

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AB-06Mix the sealerFor fresh gypsum plaster on any wall type, mix 1 part PVA to 5 parts water in a clean bucket and stir well. For old lath and plaster, use a proprietary plaster sealer or size mixed to the label instructions instead, since PVA can be too aggressive on older lime plaster. Mix only as much as you'll use in one session.

1 each · PVA adhesive, 1L

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1 each · Plaster sealer/size (for lath and plaster), 2.5L

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1 each · Mixing bucket

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AB-07Apply the sealerUse a wide brush to cut in around edges, corners and fittings, then a roller for the flat areas. Work in sections about 1m wide, keeping a wet edge so you don't get patchy overlap marks. Apply one even coat — don't let it pool or run, especially into any socket or switch fronts.

1 each · Decorator's brush, 100mm

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1 each · Roller and tray set

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AB-08Let it dry and check coverageLeave the sealer to dry fully — usually 2-4 hours, but check the label. Run a wet finger over a small area; if it still drinks the water in fast, the plaster is very porous and needs a second thin coat once the first is dry. Once dry, the surface should feel slightly stiff and even, not sticky or chalky.

AB-09Clean up and check the room is readyWash brushes and rollers in warm water straight away, before the sealer dries on them. Remove and fold away the dust sheets, and wipe any splashes off the skirting or floor. Check the whole surface looks uniform with no shiny or dull patches before you start wallpapering.

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