All you need to do is dilute some emulsion paint with water and apply it to the plaster using a roller, paintbrush or spraying. One mist coat should suffice, but you can apply a second (or even third) coat just for extra peace of mind. That way, you can rest easy knowing that the plaster is sealed.
Paint with a smooth motion to give even coverage across the wall. You'll usually only need to apply one mist coat, but you can apply a second if you don't feel you've properly sealed the plaster. If you're using a primer, always follow the instructions on the packaging.
New plaster should be painted first using a mist coat, which is a mixture of paint and water, mixed at the ratio of 4 parts paint to 1 part water. After the mist coat has dried you should then be able to apply your top coat.
Next the plaster is going to need a sealer coat applied, as new plaster is very porous. The traditional method of doing this is to thin out your 1st coat of emulsion by about 30 to 40% with clean water, making a sealer coat. This will penetrate deeply into the surface stopping any flaking off later on in years to come.
On new plaster, the first coat needs to penetrate the surface to bond. If the first coat is not watered down it will almost certainly be too thick to do this, so will not penetrate and just lay on the surface, which is a dusty surface. In time the paint will peel off leaving a horrible mess.
First and foremost, let the plaster dry. Paint won't adhere properly to wet plaster, so make sure it's fully dried. This takes around 3-5 days for overskim or 4-6 weeks for a new plaster wall, though it can take longer depending on humidity and ventilation.
The new plaster will be very absorbent and will suck the paint into the surface. Thinning the first coat with 10% clean water will help but the second coat will go much further.
Too Much Water
You don't want to be too thin, but a mist coat needs to be thin—almost like a soup. The water will not seep into the substrate if the mist coat is too watery.
There are a wide range of reasons paint can peel, including the surface being damp, salt migrating through the plaster causing the paint to peel or the surface not being completely clean before painting.
Newly plastered walls need sealing in the same way as your ceiling or drywalls: they should have a stabilising primer layer first, before they're applied with at least two coats of paint. A previously painted wall should also have at least two coats, without the primer layer first.
In good conditions – typically in summer months – this will usually take around three days. However, we'd recommend leaving it for at least two weeks if possible to be safe. In the wrong conditions, plaster can even be known to take up to four weeks to dry!
For instance, if the plaster you're painting is thoroughly dry, it's a good idea to thin down the paint. It makes paint goes on better, and it will soak into the plaster better. To thin down emulsion ready for spraying, the best ratio is 70/30. That is 70% of paint and 30% water.
While there is a great deal of conflicting advice on which ratio of water to paint is best for painting plaster, we advise anything from two parts water to eight parts emulsion paint, down to 50/50 to ensure success – check the manufacturer's tin for the requirements for your specific brand and feel free to add more ...
Applying the second coat too early will result in lost time, money, and effort. Not only that but it'll also create additional stress that's not necessary. Applying the second coat too early will result in streaks, peeling paint, and uneven color.
Painting a newly plastered wall requires more preparation than painting a normal wall. You can't start painting new plaster directly and instead have to take steps to seal the plaster so you can get the colourful finish you want.
Put simply, a mist coat is the first coat of paint that's put on bare plaster. Think of it as a base that's used to prime and seal the wall. Without the mist coat, the plaster will soak the moisture out of the paint like a sponge, causing it to crack as it dries.
Patchy paint can result if paint has been applied unevenly. Patchy paint can be down to a variation in surface absorption. Walls might appear patchy if they have variation in texture. Using paint that is not fully mixed can cause a patchy appearance.
So watering down your first coat as a 'mist coat' will help the plaster to soak up the paint and create a bond between the new plaster and the paint. For new plaster it is critical that you start with a mist coat, which leads me quite nicely into my next section… what will happen if you don't mist coat!
Mix your mist coat paint
Mist coat paint is simply watered-down emulsion paint, which acts as your primer. All you need to do is get some white emulsion paint and thin it out with water – it's that easy! Ten parts paint to one part water should work.
It should be thin enough to absorb into the plaster but not so watery that it needs colour and coverage. Test the mixture: if possible, test it on a small, inconspicuous plaster area to check its absorption and drying. Mixing a mist coat is a straightforward but essential process in painting new plaster.
A mist coat should be left to dry for around 24 hours, even if it feels dry before this. Only one mist coat should be needed, but if you have some paint leftover that's already been watered down, you might want to add a second mist coat. As long as the first coat was dry, two mist coats will do no harm.
When a painting contractor waters down paint, they are typically adding a significant amount of water to it. Think two gallons of water for every five gallons of paint. In contrast, the process of thinning paint involves a relatively small amount of water: 2-4 cups for every five gallons of paint.
Recoat Time for Oil-Based Paint
Water-based paints dry considerably faster than oil-based paints because the water evaporates to let the paint dry. Oil-based paints contain no water. Though oil-based paint may feel dry after only a few hours, it's best to wait a full 24 hours or more before applying additional coats.
Applying the second coat of paint before letting it adequately dry will result in your paint peeling, clumping, and bubbling. It can also result in a different colour than what you intended.