Painting new plaster needs a sealing layer known as a mist coat. Here we reveal the perfect mist coat ratio, plus a tutorial for painting newly plastered walls
(Image credit: Luke Arthur Wells)
Painting new plaster isn't just simply a case of slapping on a coat of emulsion. New plaster gives a nice smooth finish to walls and ceiling which makes it the ideal surface for painting, but painting directly onto plaster can cause issues that will lead to paint not adhering and flaking off.
If the plaster is not dry the paint will also trap the moisture left in the plaster which can lead to damp issues and cause the paint to crack and peel.
Say hello to a mist coat. This is effectively watered down emulsion that soaks into plaster to create a bond, acting like a primer, to ready the plaster for the top coat.
It's an essential part of how to paint a room that hasn't already been decorated or has been freshly plastered, but luckily any competent DIYer will be able to tackle the job and apply with a brush or roller.
How long should I wait before painting new plaster?
Before painting new plaster, it needs to dry properly, but it's not a quick-drying material. In ideal conditions, typically the summer months, it can take from two to three days to two or three weeks.
Ideally leave it for at least two weeks to make sure it is completely dry. Leave a window open to help ventilation, this will help speed up the drying process. One way to gauge if the plaster is dry or not is by the colour. You will see lighter colour patches appear — this means it's dry. When the plaster is all the same lighter colour the plaster is dry.
If you apply paint before the plaster is dry this can lead to other problems in the future so if you want a good long-lasting paint job you need to be patient.
New plaster needs to have dried evenly before a mist coat can be applied. (Image credit: Luke Arthur Wells)
What is a mist coat?
New plaster is very permeable , which means it will absorb liquid i.e. paint. To stop this happening a mist coat is applied directly to new, fully dry plaster to help seal it ready for the top coat. This is effectively a watered down emulsion. One coat should be enough, but you can add a second or even third coat to make sure the plaster is sealed.
Leave the mist coat for 24 hours to dry. A simple test to see that the mist coat has worked is to use masking tape. If it comes off the wall clean, you're ready to go.
What is the ratio for a mist coat?
A mist coat needs to be absorbed by new plaster so it needs to be pretty thin, almost like a thin soup. If it’s not thin enough it won’t do its job properly.
Use a white emulsion (it doesn’t need to be high quality) as this provides a good base colour for any top coat. A common ratio is 7 parts paint and 3 parts water, but you can go 50-50 if the paint you are using is thick to start with. It's better to be too thin than too thick.
Many standard white emulsions will include information about the best ratio for using them as a mist coat on the product packaging, so if you're unsure if your paint is thick or thin, consult this before mixing your mist coat.
Many white emulsion paints will have a mist coat ratio printed on the tub. (Image credit: Luke Arthur Wells)
What is the best way of painting new plaster?
1. Preparation
Before you start any painting you need to make sure that the plaster is completely dry. Before you apply the mist coat make sure that the plaster is smooth. Use sandpaper - or a sander - to get rid of any imperfections. Prep the area by laying down dust sheets, cover skirting boards with painters tape and cover any furniture. Adding a mist coat can get messy.
2. Mix your mist coat
Matt white emulsion is the easy choice for a mist coat. It's cheap and works with all colours. But you can use a coloured emulsion that is closer to the top coat to a better match. To mix the mist coat get a clean bucket and pour in the emulsion. Use a measuring jug for accuracy and don't go past halfway, now add the right amount of water and mix together.
(Image credit: Luke Arthur Wells)
3. Apply your mist coat
The quickest option is a roller. This will be more messy than a brush but gives a good even finish. You can use a paint sprayer, but unless you are a pro stick with the brush and roller.
Start in a top corner with a brush — the best paint brush for the job will be a 4-inch one. Paint along the top of the wall to the opposite corner. Now go along all the edges before getting out a paint tray and a roller.
(Image credit: Luke Arthur Wells)
The best paint roller for painting new plaster is 9-inch roller. Pour the mist coat into the paint tray and coat the roller with paint, but don't overload. Using a smooth even motion start at the bottom and roll to the top until the wall is covered. Don’t roll over the same spot too often.
Once finished, leave to dry for 24 hours, check there are no damp spots. Once completely dry apply the top coat — typically you will need two.
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Steve Jenkins is a freelance content creator with over two decades of experience working in digital and print and was previously the DIY content editor for Homebuilding & Renovating.
He is a keen DIYer with over 20 years of experience in transforming and renovating the many homes he has lived in. He specialises in painting and decorating, but has a wide range of skills gleaned from working in the building trade for around 10 years and spending time at night school learning how to plaster and plumb.
He has fitted kitchens, tiled bathrooms and kitchens, laid many floors, built partition walls, plastered walls, plumbed in bathrooms, worked on loft conversions and much more. And when he's not sure how to tackle a DIY project he has a wide network of friends – including plumbers, gas engineers, tilers, carpenters, painters and decorators, electricians and builders – in the trade to call upon.
