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How to... prepare walls

Walls almost always need some surface preparation before you can apply paint. Even brand-new plaster needs sealing. Cracks and holes must be filled, and surfaces must be clean, smooth and dry. Emulsion-painted surfaces in good condition can be painted over, however peeling paint should be stripped back or sanded first. Only paint over wallpaper if it is very firmly stuck to the wall.

What you'll need

Before you begin any preparation or decoration work, clear the room as much as possible. Anything that has to stay should be moved into the centre of the room and covered with dust sheets. Use dust sheets to protect the floor too. Soaking wallpaper with hot water makes it much easier to remove. The water soaks in more easily if you perforate the paper first, either by scoring with the blade of a stripping knife or running an orbital scorer over the surface. Be sure to stand on a stable stepladder to tackle the higher sections.

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  • Step 1

    Run the orbital scorer over the paper, being careful not to damage the plaster behind. Fill a bucket with hot water and add some liquid detergent and a handful of wallpaper paste to thicken the water, so it doesn't run down the wall quite so fast.

  • Step 2

    Using a large sponge, wet the wall, covering a few square metres at a time. Leave it to soak for at least five minutes. Slide the edge of a wide stripping knife under the paper at a seam to see if it is ready to be stripped. It should come away in fairly large sections. If this doesn't happen even after a long and thorough soak, you will need to use a steam stripper.

  • Step 3

    Hold the stripper pad at the bottom of a wallpaper length for a minute or so, until the paper around it appears damp. Move the pad up onto the next section, while pulling off the damp paper with the other hand, loosening stubborn areas with a stripping knife. Strip each length from bottom to top. Take care not to damage the plasterboard or plaster behind the paper with the blade of your stripping knife.

  • Smooth surfaces

    The difference between a good paint job and a disappointment, is almost always down to the time and care spent in preparing the surface. It is often tempting to get quickly to the excitement of the final coat, but no paint can properly mask dirty or uneven surfaces. Even on new surfaces there may be small areas that need touching up. If a surface has been previously painted, you will need to remove or fill chips, blisters and bumps and strip back peeling paint. Be sure to allow any fillers to dry completely before carrying on. Apply the filler with a filling knife. Then wait for the specified time for the filler to dry. Sand the filler back with sanding paper and a sanding block. Finally, use your hand to test the smoothness of the finish.

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  • Washing

    Painted and wallpapered surfaces alike need to be washed down before being painted over. The less dirt or grease on a surface, the better the final paint job will look. Grease, damp, mould, nicotine stains, children's drawings and finger marks can all be removed using detergent or, better still, sugar soap. Wear waterproof gloves and safety goggles as you work, as sugar soap can irritate the skin. Always finish by rinsing with clean water and then leave to dry completely.

    In older houses you still sometimes find distemper. This old fashioned emulsion is often dusty or powdery to the touch, and will rub off as you wash the surface. Don't be tempted to paint or wallpaper over distemper, as neither will stick. You must wash and scrape off as much as possible, and then seal with a special stabilising solution.

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  • Sanding down

    Sanding creates a smooth, even foundation that greatly enhances the final finish of paintwork. It gives a slight roughness – a ‘key' – to surfaces such as varnish or paint, and this helps the primer or paint to stick. On stripped plaster, sanding levels out any repairs and also removes stubborn traces of old wallpaper or paste. When sanding wood, remember always to work in the direction of the grain.

    Sanding produces a lot of dust, so be sure to protect yourself with a dust mask and good ventilation, and to cover anything vulnerable with dust sheets. Once finished, you will need to dust down the whole sanded surface from ceiling to floor, and vacuum thoroughly. It may sound like a lot of extra work, but the professional finish sanding creates really does make it all worthwhile in the end.

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Primers for different surfaces

Below is a break down of the different primers needed depending on the condition and the surface material you are working with.

Previously painted surfaces

You don't need to prime a surface that has previously been painted and is in good condition: washing and light sanding are usually enough. But you will achieve a better finish on most other surfaces by priming them first. Paint will cling firmly to a coat of primer, and not crack or flake.

Stripped surfaces

If you have stripped a previously painted surface back to the bare material, priming the exposed area will give a smoother final finish.

Porous surfaces

On a porous surface, primer prevents the top coat from being absorbed, meaning you have to apply fewer coats of paint to achieve good coverage.

Non-Porous surfaces

On non-porous, shiny surfaces, paint often won't bond properly: a coat of primer gives it something to stick to.

Note:

Different surfaces need different primers, so be sure to select the right one and to follow the manufacturer's instructions carefully. If you are in a hurry, look out for quick-drying primer: some can be painted over in just an hour. Water-based primers are low odour and less damaging to the environment.

Surface Primer
Wood Use a wood primer. This fills and levels the wood grain, and prevents the paint soaking in. Ensure wood is dry before applying.
Metal Use a metal primer. This is anti-corrosive and helps paint to bond to the smooth surface of the metal. Different primers may be required for ferrous and non-ferrous metals. Special primers can be used to prevent rust or can be painted on to rusty surfaces.
Radiators Bare radiators or exposed areas should be primed using a metal or radiator primer. This primer helps prevent metal corrosion and improve paint finish.
MDF MDF is highly absorbent; the primer seals the porous surface, helping prevent paint soaking in.
Bare Plaster Plaster sealer will seal the porous surface, preventing paint from sinking in. Without it, you will need many more coats of paint. Be sure to allow new plaster to dry out completely first. This can take up to four weeks for a skim coat and several months for a full new plaster. As it dries, the surface of the plaster will turn from dark to pale pink.
Multiple surfaces If you have lots of small areas to prime, a multi-surface, or 3 in 1, primer can be very useful. Suitable for use on wood, metal or plaster, it will save you buying lots of separate products.
Tiles, melamine, PCVu Use a specialist primer to ensure that paint bonds to the smooth hard surface: tile primer, melamine (or cupboard) primer, or PCVu primer, as appropriate.
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