Wall Paint Primer and Undercoat: Types and Prices

A wall paint primer is the coat you never see, and it is often the reason a paint job lasts or peels. Primers seal porous plaster, bind chalky surfaces, stop rust on steel and help paint grip on wood and shiny old coats. Undercoats build an even base so the top coat covers in fewer coats. Choosing the right one for the surface matters more than the top coat brand. Tell us the surface and suppliers near you quote on primers and undercoats. It is free and you are under no obligation.

Which primer for which surface

Plaster primer for new plaster once it has cured (often about four weeks, or ask your painter), and for porous or slightly powdery plaster.
Alkali-resistant primer where new plaster or cement is still alkaline, so the top coat does not discolour or peel.
Bonding liquid on chalky old paint and dusty surfaces, after brushing down, to bind the surface.
Metal primer for steel: a rust-inhibiting primer on wire-brushed rusted steel, and a primer made for galvanised steel on new gates and roof sheeting.
Wood primer on bare timber doors, frames and fascias.

Undercoats

A universal undercoat goes over primed wood and metal, and over old enamel after sanding, to give a smooth, even base for gloss or enamel paint. It also helps when you change from a dark colour to a light one. On walls, a tinted undercoat or a coat of plaster primer often means fewer coats of the more expensive top coat.

Prepare first, then prime

Primer only works on a sound, dry surface. A lot of peeling paint comes from priming over dust, chalk or damp. Scrape loose paint, wash off dirt and grease, treat mould, brush off efflorescence and find its cause, and fill cracks. Do the chalk test: rub the wall with your hand, and if it leaves white powder, it needs bonding liquid. Fix leaks and rising damp first, because primer does not cure damp. For the top coats, see wall paint and exterior paint.

What it costs

Suppliers quote plaster primer at about R1,050 to R1,270 per 20 L, bonding liquid at about R310 to R420 per 5 L, and undercoat at about R450 to R1,100 per 5 L, depending on the grade. Metal and wood primers are usually quoted per tin by type. Skipping primer to save money is a false saving: paint that peels off a chalky wall has to be scraped and redone. Buy primer for the actual surface, not a general one, and check the recoat time before the top coat goes on.

What to include in your request

The surface: plaster, steel, galvanised or wood
Whether the old paint is chalky or peeling
Roughly how many m² to prime
Tin sizes you prefer
The top coat you will use over it

Primers and Undercoats for different clients

Homeowners: bonding liquid on a chalky old outside wall before a fresh top coat
Landlords and sellers: primer on filled patches and stains so a quick repaint looks even
Body corporates and estates: bulk plaster primer and metal primer for walls and gates in a complex repaint
Shops, offices and restaurants: a primer to cover strong old corporate colours before a new scheme
Factories, farms and mines: rust-inhibiting primers on steel structures and galvanised roof sheeting
Builders and developers: plaster primer on new walls once they have cured, before final coats
Schools, churches and public buildings: primer on patched and repaired walls in schools and clinics
Painters and contractors: plaster primer and undercoat by the 20 L tin on a trade account

Frequently asked questions

Do I need a primer before painting a wall?

You need one on new or bare plaster, chalky or powdery walls, patches of filler, bare wood and bare steel. A sound, clean wall that was painted before with a similar water-based paint can often go straight to top coat. If in doubt, do the chalk test and ask your paint shop.

What is the difference between a primer and an undercoat?

A primer is the first coat on a bare or problem surface: it seals, binds or protects against rust so later coats grip. An undercoat goes on after the primer, mainly under gloss and enamel, to give an even, smooth base and help the colour cover. Some jobs need both, some only a primer.

How long should new plaster cure before priming?

Let new plaster cure fully before you prime, often about four weeks, or ask your painter. Weather, plaster thickness and ventilation all change the time. Painting too early can trap moisture and alkali, which leads to blistering and patchy colour. An alkali-resistant primer helps where you cannot wait long.

Can I get primer quotes from several shops at once?

Yes. We are a free quote service: list the primers and undercoats you need, with tin sizes and the surfaces, and paint shops and suppliers near you quote on it. Ask each to confirm the product suits your surface, so you compare like with like rather than only the price.

The primer under the paint decides how long it lasts

Which primer for which surface

Undercoats

Prepare first, then prime

What it costs

What to include in your request

Primers and Undercoats for different clients

Frequently asked questions

Primers and Undercoats by province and city

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