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Should I prime before I paint — and how much primer do I need?

Always prime new, absorbent, stained or glossy surfaces. Primer covers 8-12 m² per litre and typically saves a whole coat of expensive topcoat.

What primer actually does

Primer solves three different problems, and each has its own product. That's why the question "should I prime?" doesn't have one single answer — it depends on what's wrong with the wall.

• Absorption: a porous surface drinks up the first coat of paint. The result is patchy areas where absorption is greatest, and consumption 30-50% above what the tin promised. A deep-penetrating primer saturates the pores, so the topcoat sits on top of the surface where it can be seen.
• Bleed-through: water stains, soot, nicotine, marker pen and knots in pine timber work their way up through coat after coat of ordinary paint. A stain-blocking primer seals them in. Ordinary paint won't, no matter how many coats you add.
• Adhesion: new paint slides off a glossy, smooth surface. A bonding primer gives it something to grip on old glossy paint, laminate, tiles and metal.

When should you prime — and when not?

Always prime

• New plasterboard and freshly filled patches. Uneven absorption is the most common reason a newly painted wall looks patchy in raking light.
• Bare render, concrete, aircrete and brick.
• Bare wood. Knots bleed yellow through — they need stain-blocking primer, otherwise you can paint three white coats and still see them.
• Water stains, soot or nicotine. Stain-blocking primer over the whole surface, not just the stain — otherwise you'll see where the stain was, as a difference in sheen.
• Old, glossy paint on doors, frames and trim. Sand it matt first, and use a bonding primer if the surface is still smooth.

You can safely skip it

• The wall has been painted before, is matt, clean and sound, and you're painting the same or a similar colour.
• The door is factory-primed from new — it's already primed; a light sanding and two coats of topcoat is enough.
• You're using a paint whose own instructions say it's self-priming on that surface. The tin's instructions always take precedence over a rule of thumb.

How much primer do you need?

Primer goes on in one coat, not two. Coverage is typically stated as 8-12 m² per litre, but on a strongly absorbent surface it covers less — allow for the low end, and as little as 6-8 m² per litre on new, bare render.

Example, one room: 4.20 × 3.60 m with a 2.50 m ceiling gives 34 m² net wall area (wall area minus door and windows). 34 ÷ 10 = 3.4 litres. Allowing for waste: 3-4.5 litres of primer — so a 5-litre tin.

Example, a whole flat: 160 m² of wall ÷ 10 = 16 litres. If the ceilings need priming too, add another 75 m²: a further 7-8 litres. 20-25 litres in total for an 80 m² home, if everything is new.

The arithmetic that shows why primer pays off

Take a room with freshly filled walls, 34 m² net, and count litres instead of money.

Without primer: the absorbent surface really needs three coats of topcoat before the finish is even. 34 × 3 = 102 m², and the first coat soaks in extra, so allow 6 m² per litre: about 17 litres of topcoat — and three times the work.

With primer: 3.5 litres of primer plus two coats of topcoat: 34 × 2 = 68 m² ÷ 7 = about 10 litres of topcoat — and two times the work.

So you're trading seven litres of the most expensive paint and a whole coat of labour for three and a half litres of the cheapest. The only cost is an extra day of drying time. On a surface that doesn't absorb, the arithmetic runs the other way: there, primer is an unnecessary coat.

Four traps that cost you paint

• Primer isn't topcoat. It's made to bond and saturate, not to look good or stand up to washing. Paint always has to go on top.
• Stain-blocking primer has to cover the whole surface. Prime just the stain, and the difference shows up as a change in sheen when light falls at an angle.
• Primer has to dry fully. Paint over it too soon, and the bleed-through can still come through — especially with nicotine and water stains.
• You can't "prime" with thinned-down topcoat on a surface with bleed-through. Thinned paint saturates absorption, but it doesn't block anything. They're two different jobs.

How long does primer need to dry?

Drying time is on the tin and isn't negotiable — especially for stain-blocking primer. As a rough guide, water-based primer is paintable over after 2-6 hours, while stain-blocking primer against water stains, nicotine and knots often needs 6-24 hours. Paint over it too soon, and the topcoat can soften the primer, letting the stain bleed through anyway.

The drying time also assumes ordinary room temperature and normal humidity. In a cold, damp room — an unheated basement or a newly renovated bathroom — it can easily double. Air the room out, keep it warm, and don't trust that the surface feels dry: the top dries long before the layer underneath.

Primer shouldn't sit too long before it's painted over either. Some products state a window — for example paintable after 4 hours, but no later than a couple of weeks — because adhesion drops once the coat is fully cured and has gathered dust. Read the tin's instructions; they always take precedence over an article.

The surface the figure rests on

Both primer and topcoat are worked out from the same surface: the net wall area — the room's perimeter times the ceiling height, minus doors and windows. HouseSense calculates it from the home's own geometry, room by room, whether the home was scanned with the iPhone's LiDAR or drawn on graph paper in the app. From there, both primer and topcoat come down to the same small piece of arithmetic: area ÷ coverage, times number of coats, plus waste.