Wondering which primer paint you need? There is no single best primer, only the right one for the surface in front of you. This guide explains what primer does, the main types of primers for paint, which to use on wood, plaster, masonry, metal, concrete and plastics, and how to prep, apply and buy the right amount.
What is Primer Paint?
Primer is a preparatory coating applied before the topcoat. It seals porous surfaces, evens out absorption, improves adhesion and, on metal, inhibits rust. Paint and primer do different jobs: primer readies the surface, and paint adds colour and protection. Skipping it is a common cause of peeling, as our guide to the role of primers and undercoats explains. The British Coatings Federation, the UK coatings trade body, covers how paint systems are designed to work together.
A quick note on terms, because they get mixed up. A primer bonds to the bare surface. An undercoat sits on top of the primer to build opacity and a smooth base for the colour. A sealer locks down porous or dusty surfaces and is often used on concrete and masonry. Our guide to the difference between a primer and a paint sealer explains which you need.
Which Wood Primer should I use?
Bare timber is porous, with resins and knots that can bleed through paint. A wood primer seals the grain so the topcoat sits evenly and lasts. Treat knots with a knotting solution first, or use a stain-blocking primer. Indoors, a water-based acrylic primer dries fast and suits most softwoods. Outdoors, choose an exterior wood primer labelled for external use. A paint primer exterior wood can rely on must cope with movement, moisture and UV. Weathered timber should be sanded back to sound wood before priming. For the finish, browse the wood care range.
When should I use an Oil Based Primer?
An oil based primer is the traditional choice for difficult surfaces. It blocks stains and knot bleed, grips glossy or previously painted surfaces, and gives a hard base for solvent-based topcoats. It dries more slowly and smells stronger, so ventilate well. A paint primer oil based formula is often safest on weathered timber, stained surfaces and bare metalwork. For everyday interior work, a water-based primer is quicker and lower in odour.
What primer paint for plaster do I need?
New plaster is very absorbent and soaks up full-strength emulsion unevenly. A mist coat or dedicated plaster primer seals the surface so the topcoat dries evenly. A mist coat is simply emulsion thinned with water, usually around 30 to 50 percent, applied once the plaster has fully dried out. When you prime and paint walls that were previously painted, sound emulsion needs no primer, but do prime painted walls that are glossy, stained or patched. Water or smoke stains need a stain-blocking primer, and humid rooms benefit from a primer or paint with a biocide, such as the anti-condensation range.
Which primer should I use on metal?
Metal is the one surface where primer is never optional. Bare steel and iron need an anti-corrosive oxide primer that seals out moisture and oxygen before rust starts. The Metal Oxide Primer Paint from the oxide primers range gives a rust-inhibiting base for a solvent-based topcoat. Galvanised steel and aluminium are different: their smooth, non-ferrous surfaces resist paint, so degrease thoroughly and use an etch primer or a primer specifically rated for galvanised metal, otherwise the topcoat will flake.
Do concrete floors need a primer?
Yes. Bare or porous concrete should be sealed with a concrete floor primer before floor paint goes on. It stops the slab drinking in the first coat and gives the topcoat something to bond to, the biggest factor in preventing peeling. Garage floors, workshops and patios all follow the same rule. If you are starting from scratch, our guide on whether you can paint concrete walks through the full process, and if a previous coating has already lifted, our guide to why floor paint peels explains how to put it right.
Can you prime plastics, uPVC and MDF?
Yes, with the right product. Glossy plastics and uPVC resist standard primers, so clean them first with a dedicated uPVC cleaner, lightly abrade, and use an adhesion primer made for plastics. MDF is extremely absorbent, especially on cut edges, so seal it with an MDF primer or a couple of thinned coats before the topcoat, or the edges will stay rough and dull.
How do I prep a surface before priming?
Whatever the surface, the primer coating paint needs a clean, dry, sound base. Follow three steps:
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Clean. Wash interior surfaces with sugar soap and exterior surfaces with a suitable cleaner, treat any mould with a fungicidal wash, and let everything dry fully.
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Repair and sand. Fill cracks and holes, sand repairs flush, and lightly abrade glossy areas so the primer has a key. Dust off afterwards.
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Protect and ventilate. Mask edges, lay dust sheets and open windows, especially with oil-based products.
Before painting, prime only once those three steps are done, then apply one even coat.
How much primer do I need and how long does it take to dry?
Primer coverage is printed on the tin, typically around 10 to 14 square metres per litre on smooth surfaces and less on bare, porous ones. Measure your area, divide by the coverage rate, and buy about 10 percent extra for porous patches and edges. Most water-based primers are recoatable in two to four hours, while oil-based primers usually need overnight. Apply between roughly 10 and 25 degrees Celsius and avoid damp or humid conditions, as our guide on drying time vs curing time explains. Always follow the recoat time on the tin.
