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Do you need a primer before painting and which one to choose
When a primer is essential and when you can skip it: deep-penetrating, concrete contact, water-repellent, metal and wood primers under enamel, how many coats and drying times.
When a primer is needed
A primer evens out the absorbency of the surface, binds a dusty layer and improves paint adhesion. It is essential on fresh plaster and filler, plasterboard, highly absorbent substrates (aerated block, old lime plaster), chalking old paint, glossy surfaces, bare metal and wood, and when changing from a dark colour to a light one.
Without a primer, filler draws water out of the paint unevenly and patches show on the wall, while paint consumption goes up. You can skip the primer when repainting sound paint of the same type: just wash the surface and fill any damage.
Deep-penetrating primer
Acrylic deep-penetrating primer is the most common one: it is for mineral substrates such as filler, plaster, plasterboard, concrete and brick. It soaks into the pores, strengthens the surface and reduces absorbency. Consumption is about 100–200 ml/m² per coat depending on the substrate; ready-to-use primer is sometimes diluted with water as the label says, concentrate always.
Apply with a roller or brush without puddles: excess primer forms a glossy film on which paint holds worse. It dries in about 1–4 hours, and painting usually starts the next day. Universal acrylic primer does the same job on less absorbent substrates.
Concrete contact: when and why
Concrete contact is an adhesion primer with quartz sand: it makes a smooth, non-absorbent base rough. It is used on cast concrete, old tiles and painted walls before plaster, filler or tile adhesive. It is usually pink so that missed spots are visible; consumption is about 250–350 g/m².
Concrete contact is not used directly under paint, because its rough texture would show through. It needs about 12–24 hours before the next layer goes on.
Water-repellent and facade primers
For facades, plinths and damp rooms there are facade and water-repellent (hydrophobic) primers. Silicone and silicate facade primers strengthen plaster and prepare it for a facade paint of the same type. A hydrophobic primer keeps brick, stone and concrete from soaking up water while letting vapour through, so mould and salt stains appear less often. In damp rooms, bathrooms and kitchens choose a primer with an anti-mould additive.
Primer under enamel: metal and wood
Bare metal is always primed: with an alkyd anti-corrosion primer (GF-021, for example) or a 3-in-1 paint that combines primer and enamel. First remove rust with sandpaper or a wire brush and degrease the surface with solvent. For small parts an aerosol primer is convenient.
For wood use an alkyd primer or a first coat of the same enamel thinned by about 10 %. Knots and resin stain light enamel, so they are sealed with an isolating primer. Plastic and glossy surfaces need an adhesion primer, otherwise the enamel will peel.
How many coats and how long it dries
One coat is usually enough; on a highly absorbent base (aerated block, old lime) apply two, the second after the first has dried. A simple check: splash water on the primed wall; if it soaks in at once, another coat is needed. When going from a dark colour to a light one, a white or tinted primer helps.
Drying: acrylic primer 1–4 hours, painting after 4–24 hours; alkyd GF-021 about 24 hours; concrete contact 12–24 hours. Work is normally done above +5 °C. ASB Trade stocks POLUS and other primers, fillers, rollers and brushes, from a single can at wholesale prices.
Frequently asked questions
Can I paint over filler without a primer?
Better not: filler draws water out of the paint unevenly, patches appear, paint consumption rises and sometimes the paint peels off together with the top layer. One coat of deep-penetrating primer removes these problems.
Can concrete contact be used under paint?
No; it is for plaster, filler and tile adhesive, and its sandy texture would show through paint. On a glossy painted wall use a universal adhesion primer under paint or dull the surface with sandpaper.
How soon can I paint after priming?
Over acrylic primer usually after 4–24 hours, most conveniently the next day. Alkyd metal primer dries for about a day. The exact time is on the label and depends on temperature and humidity.