What is primer for and when do you need to use it?

Primer is a coat applied before the paint. It helps the paint stick, evens out how much the wall absorbs and covers stains or colour changes. It is needed on new walls, bare plaster, stains, metal and wood. On a wall that has already been painted, is in good condition and has no big colour change, you can often skip it.

What primer does

Think of primer as the foundation of a good job. It is not the final paint. It is a coat prepared to hold the paint and improve the result.

Improves adhesion: the paint stays fixed to the surface.
Evens out absorption: without primer, a new wall soaks up the paint unevenly and ends up patchy.
Seals stains: helps stop smoke, grease or old damp stains from showing through the new paint.
Protects the material: on metal and wood, there are specific primers that protect.

When it is needed

Use primer in the following situations.

New walls or plaster that has not yet been painted.
Plasterboard and freshly applied finishing filler.
Large repairs, where the wall has areas that are very different from each other.
Surfaces with stains that show through the paint.
Going from a very dark colour to a very light one.
Chalky walls, which leave dust on your hand when you touch them.
Bare wood, metal and difficult surfaces, such as old gloss paint or tiles, with the right primer.

Each surface needs a different primer. A plaster wall uses one type. Metal uses another. Tell the shop what you are going to paint.

When you can skip it

If the wall is already painted, clean, firm and free of stains, and is going to stay a similar colour, primer is not usually necessary. Just clean, repair small flaws and paint.

Some paints say they already include primer. These paints help in some cases, but on new or very difficult walls, a separate primer is still the safest option.

Primer and changing colour

When you go from a dark shade to a light one, the old colour can show through the new one. A light primer helps cover it. Even so, you may need another coat of paint.

Going the other way, from light to dark, is usually easier. But some primers can be tinted to be closer to the final colour. Ask for advice in the shop.

How to apply

Apply the primer on a clean, dry, dust-free wall. Use a roller or brush and spread it evenly, without runs.

Respect the drying time given on the tin before painting over it.
Do not leave areas without primer, especially patches and repairs.
Apply in normal temperature conditions and without excessive damp.
Wipe the dust off sanded surfaces before applying.

If the primer is applied to only part of the wall, as with patches, the final paint may have a different sheen or texture in those areas. If that happens, apply an extra coat of paint over the whole wall.

Tip: Even when the shop says you do not need primer, do a small test in a corner. If the final paint does not cover well or ends up patchy, primer will save you work.