Plaster primer is an alkali-resistant primer for new plaster and cement walls that neutralises the surface and seals it so the topcoat bonds and lasts. Fresh plaster is highly alkaline and porous, and paint applied straight onto it burns, flashes patchy and peels. Plaster Primer is the high-performance first choice for new walls; MultiPrime is the budget primer-and-topcoat option. This guide explains what plaster primer does, when you need it, and how much to buy.
What does plaster primer do?
Plaster primer neutralises the alkalinity of new plaster, seals the porous surface, and gives the topcoat a sound, even base to bond to. New cement plaster is chemically alkaline and drinks paint unevenly. Left unprimed, that alkalinity attacks the topcoat (a fault called burning or saponification), and the porosity makes the finish patchy. An alkali-resistant plaster primer stops both problems: it holds back the alkalinity and evens out absorption, so the paint goes on evenly and stays put.
Do you need a primer on new plaster?
Yes, new plaster and cement walls need an alkali-resistant primer before painting, without exception. This is the one surface where skipping the primer almost always fails: raw plaster is too alkaline and too porous for a topcoat to hold. Whether the wall is getting a budget acrylic PVA or a premium finish, it needs the primer first. The only question is which primer, and how long to let the plaster cure before you apply it.
How long should new plaster cure before priming?
Let new plaster cure for about 28 days before priming and painting, so it dries out and its alkalinity settles. Painting green plaster — plaster that is still curing — traps moisture and leaves the alkalinity at its most aggressive, which is a recipe for peeling and patchiness. The 28-day rule is a working guide; in cold or damp weather plaster takes longer. If you must paint sooner, an alkali-resistant primer is even more important, but the safe course is to wait for the plaster to cure.
What primer is best for plaster?
The best primer for new plaster is a high-performance alkali-resistant plaster primer; a multi-surface primer-and-topcoat is the budget alternative. Plaster Primer (the alkaline plaster primer) is the first choice for a lasting result on new walls. MultiPrime is a multi-surface primer that also works as a matt topcoat, which makes it the economical option for walls and ceilings where a separate primer and finish are not needed. For chalky or powdery old plaster rather than new plaster, use a bonding primer — see bonding liquid.
Solvent-based or water-based plaster primer?
A solvent-based plaster primer gives the strongest alkali resistance and adhesion on new plaster; a water-based primer is lower-odour and easier to clean up. Plaster Primer is the solvent-based, high-performance choice for a durable seal on demanding new-plaster jobs. Where low odour and water cleanup matter more, a water-based primer is the easier option. On a new home or a re-plastered wall that you want to paint once and forget, the solvent-based primer is the safer specification.
How do you apply plaster primer?
Apply plaster primer by cleaning the cured plaster, then brushing or rolling one even coat and letting it dry before the topcoat. Brush off dust and loose material first, and make sure the plaster is cured and dry. Roll or brush Plaster Primer on in one even coat, working it into the surface, and let it dry for the time on the label before the first topcoat. Then apply two coats of your chosen wall paint. A well-primed wall carries the finish evenly with no patchy flashing.
Can you paint over old plaster primer?
Yes, you can topcoat over a plaster primer that is sound and clean, even if it has been down a while, after a wipe to remove dust. Primer left exposed for a long time collects dirt that weakens the bond, so clean it and check it is still firmly stuck before painting. If the primed surface is dusty, chalky or lifting, sand it back and re-prime rather than paint over a failing coat. On a freshly primed wall, follow the recoat time on the tin and topcoat within the window for the best bond.
How much plaster primer do you need?
Budget one coat of primer at roughly the coverage of a coat of paint, allowing extra because new plaster is thirsty. Porous new plaster drinks the first coat, so a litre covers less than it would on a sealed wall. Plaster primer comes in 5 litre (L) and 20 L, with the 20 L the economical size for a full house. Measure the wall area, match it to the coverage on the product page, and add a margin for absorption. For current pricing, see the product page or the full price list.
Where to buy
Shop Plaster Primer and MultiPrime in the Primers range with national delivery, or visit our paint shops in Table View, Cape Town and Edenvale, Johannesburg. For trade pricing, call +27 84 985 6141. See the full paint primer guide.
