Without the mist coat you're likely to have very poor adhesion between your paint and your plaster and quite often it will crack, flake or even peel off in sheets in some instances!
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.
Definitely make sure you clean it off with a damp cloth (almost wet actually) - any stuck dust can cause issues for the paint adhering and may see flaking. That won't take long to dry up and can paint straight after.
The plaster will be very porous so it will suck the water out of the paint and might not dry and adhere to the wall properly. That's why you use watered down emulsion to reduce the plasters porosity -- 25% water is about right.
Applying a mist coat to new plaster is a crucial step in the painting process. A mist coat is a thinned-down layer of emulsion paint that helps to seal and prepare the plastered surface before applying the final coats of paint.
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.
Very hard plaster may be slick, reducing the adhesion of any coating. New plaster might have a high pH level, which will require a primer resistant to alkalinity. Old plaster that has become powdery. Poor surface preparation.
As a rule of thumb, plaster takes up to 4 weeks to dry completely. You can speed things up with a heater or by opening a window. Top tip: If you don't let the plaster dry and apply emulsion paint to wet plaster, it's likely to crack and peel off.
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 ...
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.
Ideally, plaster needs to be fully dry prior to painting. Fresh plaster will take around 4-6 weeks to fully dry. Fresh plaster is porous and will therefore absorb moisture. If you attempt to paint over plaster that is still damp the paint is likely to peel off.
While some may suggest plastering and painting on the same day, it's essential to allow the plaster to cure before applying paint. Waiting for the plaster to dry completely, typically for around two weeks, allows all moisture to evaporate. This helps prevent issues such as paint peeling in the future.
The very first thing you need to do when painting new plaster is to let it dry. A week should be long enough to let it completely dry out and for all damp patches to disappear. Once the plaster is dry it should be light with no dark spots and a consistent colour and appearance throughout.
In contrast to watering down paint, “thinning” paint is a regular practice for all painters and is recommended by the paint manufacturer. To “thin” paint, an exterior painter will add water to the paint bucket, increasing the paint's workability and slowing down the speed with which it dries.
As long as the plaster has fully dried, it's safe to paint onto.
The reason behind bubbling in the plaster wall
If your interior plaster wall doesn't have a damp-proof coating on top of the paint, you are more likely to face dampness issues. Here, the moisture rises through the wall, and bubbling appears. The plaster then comes out, and the powder-like mixture falls on the floor.
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!
Curing cement plaster is essential because it strengthens the wall and decreases surface cracking. Cement plaster needs at least 24 hours to cure after application. For the first seven days after plastering, the surface should remain damp. In addition, wait seven days before applying a second layer.
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.
The main reason why paint cracks is due to poor preparation on fresh plaster. Fresh plaster needs to be totally dry prior to applying paint. It also needs to be primed before adding emulsion.
To understand why paint is peeling off your plastered walls, you need to know one or two things about plaster. In summary, if you've not allowed the plaster to fully dry and/or you've not primed the dry plaster properly before painting, your wall paint will peel and flake.
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.
If the plaster is not fully dry, painting over it will trap moisture between the plaster and the paint, eventually causing the paint to flake and peel off. So, can you fix paint on wet plaster? We'd say no.
The number one cause for bubbling and peeling paint after decorating is poor preparation and contamination, according to our technical advisor Jon. “Preparation is key. Be sure that surfaces are wiped down and given a light sand where necessary.
Increasing dampness or moisture in the walls: This can be considered the primary reason for the crumbling or peeling plaster. When moisture enters the plaster, the plaster begins to get thicken and start falling apart from the wall.