Priming Walls and Ceilings

Why does paint applied straight onto a wall so often look patchy? Plaster, filler and plasterboard all absorb water at different rates. The paint soaks into one spot more than another, and you end up with differences in colour and sheen. Primer evens this out, so the paint dries uniformly and you need less of it.

Its second job is to strengthen the surface. Old lime plaster, common in older houses, and walls that have just been scraped back often crumble and shed dust. A deep-penetrating primer binds these fine particles together, so the next coat sits on a firm base and does not flake off.

Priming is especially worth doing:

• On new plaster, plasterboard, concrete and aerated concrete blocks
• After scraping off old paint or sanding a wall
• On patches repaired with filler or plaster
• Before skimming, wallpapering, decorative plaster and tiling
• Before a self-levelling compound or bonded floor covering

The principle is the same. What changes is the surface, what goes on top and how comfortable the work is.

Walls and ceilings are usually primed before painting. The ceiling is the harder part: you work overhead, the primer drips, and it is difficult to see whether you have missed a patch. Cover the floor and furniture well, protect your eyes and use a stable stepladder or a low work platform. Ceilings also tend to show water stains from old leaks, and ordinary primer will not hide them.

Walls and floors are usually primed before tiling, a self-levelling compound or a bonded covering such as vinyl or carpet. On the floor, vacuum thoroughly, avoid leaving puddles and work so that you finish at the door, so you never have to walk across fresh primer. A concrete or anhydrite screed must be fully cured and dry. With anhydrite, manufacturers usually require the surface to be sanded and a specific primer used, so check the technical data sheet.

Deep-penetrating primer

The most common type, usually an acrylic dispersion. It suits plaster, concrete, plasterboard and aerated concrete. It strengthens absorbent, dusty surfaces and is used before painting, skimming and wallpapering.

Plasterboard primer

Plasterboard has a smooth paper face that absorbs differently from the filled joints. A primer made for plasterboard evens out the paper and the filler, so the joints do not show through the paint.

Bonding primer

On smooth, non-absorbent surfaces such as old tiles, glazed ceramics or very smooth concrete, standard primer has nowhere to soak in. A bonding primer, often containing fine sand, gives adhesive or skim coat something to grip.

Stain-blocking primer

Water stains, nicotine, soot and grease bleed through new paint. A stain-blocking primer seals them in. Standard primer will not cover them.

Universal primer

A compromise for mixed surfaces. Some are also suitable outdoors or contain anti-mould additives. Bear in mind that primer is not waterproofing. In a shower or around a bath, the tiles need a proper waterproofing membrane or tanking underneath. An anti-mould additive will not solve the cause of mould if the real problem is damp or a cold spot in the wall.

You can prime a room yourself if you have a stable stepladder and some patience. The key is a clean surface and following the manufacturer's instructions. For very high ceilings that need scaffolding, leave the job to a professional.

1. Cover the floor, furniture, windows and switches, and mask the edges with tape.
2. Remove loose paint, flaking plaster and grease, and let any filler dry completely.
3. Dust the surface down and vacuum the floor thoroughly.
4. Check that the surface is dry and the room temperature is within the range the manufacturer gives, usually above 5 °C.
5. Stir the primer and dilute it with water if the data sheet says so.
6. Brush the corners, edges and areas around door frames.
7. Roll a thin, even coat over the main surface. On a ceiling, work from the window into the room. On a floor, work from the far corner towards the door.
8. Leave it to dry, usually a few hours depending on temperature and humidity, and ventilate gently without a strong draught.
9. Apply a second coat on very absorbent surfaces, then carry on with the next stage.
10. Wash your tools in water straight away.

The quickest job is a large, flat area in an empty room with a firm, clean surface. Small, awkward spaces such as bathrooms, hallways and alcoves take longer, as do lots of corners and window reveals, high ceilings that need a platform, and a furnished room that has to be covered. A surface that first needs scraping, vacuuming or repairing adds time, and very absorbent new plaster or aerated concrete often needs two coats. Specialist products such as bonding or stain-blocking primer are an extra step. When you get a quote for decorating, check whether priming is included or listed separately.

• Priming a damp or dusty surface. The primer does not soak in and simply glues the dust to the surface
• Too thick a coat. The primer forms a shiny, slippery film that paint and adhesive do not grip well. It should be brushed or rolled on, not flooded
• Wrong dilution. Primer that is too thick will not soak in, and primer that is too thin will not strengthen. Stick to the manufacturer's ratio
• Skipping plasterboard or repaired patches. The filled joints then show through the paint
• Standard primer on a non-absorbent surface. Tiles and glazed surfaces need a bonding primer
• Carrying on too early or too late. Painting onto wet primer reduces adhesion, and so does waiting so long that dust settles on the surface again

A primed surface is easy to check: run your palm over the wall and flick a few drops of water onto it. On a well-primed surface the water sits for a moment rather than disappearing straight away.

• The surface is evenly matt, with no shiny patches or runs
• No dust or crumbs come off when you run your hand over it
• Corners, edges, reveals and areas around door frames are covered
• Filled patches and plasterboard joints are primed along with the rest
• The floor has no puddles or sticky spots
• The room is clean, coverings are removed and tools are washed