Painting new walls is seemingly easy; there is no old emulsion to strip off, after all. What you may not know is that painting directly over new plaster can end with undesirable results. Applying a mist coat – watered-down paint applied in a thin layer – is necessary to seal the plaster on your walls and prevent paint peeling. But how much water to add? What is the best misting coat ratio?
The best mist coat ratio varies from the substrate to substrate and product to product. A 4:1 paint to water ratio generally works well for new plaster. A ratio of 1:1 may suffice for old plaster. The same applies to existing and new render. However, you should always follow the instructions on the paint tin – manufacturers may recommend other dilutions.
How To Achieve Optimal Mist Coat Ratios
Achieving the optimal mist coat ratio sounds tricky, but it really is simple. If the manufacturer doesn’t recommend the dilution on the paint tub, all you have to remember are two ratios, which apply to new or existing plaster, render, and masonry.
|New plaster||4 parts paint to 1 part water (80/20)|
|Existing plaster||1 part paint to 1 part water (50/50)|
|New render||4 parts paint to 1 part water (80/20)|
|Existing render||1 part paint to 1 part water (50/50)|
|New masonry||4 parts paint to 1 part water (80/20)|
|Previously painted masonry||1 part paint to 1 part water (50/50)|
Painting new plaster is the last step toward completing a major home improvement or building work. However, most interior paints don’t adhere well to new plaster. This is why you have to prepare the surface with a primer or a mist coat.
The latter is generally easier to achieve and less expensive since you’d use diluted paint. Not only is a mist coat cheaper, but it can also improve the end result – primers are generally white or colourless, whereas the mist coat would have the same colour as the finishing coats.
Unless otherwise specified on the tin, a good mist coat ratio for new plaster is 4:1, or 80% paint to 20% water.
Some people recommend a 70/30 ratio or even higher dilutions to save money on paint. However, a mist coat that is too watered down will evaporate before soaking into plaster and won’t give you a bond.
Existing (painted on) plaster doesn’t generally require priming unless you strip off the existing paint. Even then, the substrate will still retain some of the old paint that has already soaked into it. For this reason, you can use a runnier mist coat without compromising the end result.
While there is no general rule, a 50/50 (or 1:1) ratio would work for the intended purpose.
Render is the equivalent of plaster but applied on the external walls of your property. Render is typically more absorbent than plaster, but the mist coat ratio for new render is the same as the one for new plaster – 80% paint to 20% water.
A more diluted mist coat wouldn’t only dry too fast but it may also be ineffective. Thus, you might have to apply several layers of product to achieve a good bond.
Like plaster, existing render doesn’t require a mist coat if you want to paint over existing emulsion. On portions of wall where the old paint has peeled off, you can apply a more diluted mist coat to improve adherence.
New masonry is similar to plaster and render in that it is either too porous or too smooth for masonry paint to adhere. Prepare the mist coat as you would for render or plaster to paint new or painted over masonry walls.
PVA paints are known for their excellent filling and hiding properties. You can use PVA paint to mask interior and exterior wall imperfections, such as small cracks and holes. However, PVA paint should not be diluted and is unsuitable to use as a mist coat.
If you want to apply PVA paint to new plaster or render, you should first prime the surface with an appropriate PVA primer. Some PVA primers can be diluted, generally with no more than 15% water.
On existing plaster or render, you can paint directly over the old coat if the old paint is PVA. Otherwise, you’ll have to strip the paint, prime the wall, and apply the new coat if you want to use acrylic or latex paint.
Manufacturer Recommended Mist Coat Ratios
We already mentioned that following the manufacturer’s recommendations when preparing the mist coat is paramount. Here are the recommended dilution ratios from the most popular brands:
Dulux recommends watering down your own mist coat to paint onto fresh plaster. The dilution ratio is three parts paint to one part water.
Leyland recommends thinning down the emulsion with 10% water when painting new plaster.
Crown manufactures a variety of interior and exterior wall paints. When painting new plaster, rendered surfaces, concrete, MDF, or plasterboard, you can thin down the first coat with 15% water.
Factors That Affect Mist Coats
Too Much Water
While a mist coat has to be thin – more or less like soup – you don’t want to be too thin. If the mist coat is too runny, the water will evaporate before soaking into the substrate. This would prevent the mist coat from acting as a bonding agent, and your finishing coat will start flaking or peeling in time.
Too Little Water
Similar to a thin coat, a mist coat that is too thick won’t soak into the substrate (due to its viscosity, this time) and won’t act as a bonding agent.
One of the most frequent mistakes DIYers make when painting their walls is not waiting for the mist coat to dry and cure.
Most mist coats will become dry to touch in two hours or less. However, you should allow for a curing period of at least 24 hours. This would give the paint soaked into the substrate the time to dry completely and bind to the plaster, improving adherence for the topcoat.
Painting On Wet Plaster
Painting on wet plaster – with or without a mist coat – is always a bad idea. The water will have to evaporate, resulting in two possible outcomes: your paint will swell, crack, and start to peel off if it is not breathable, or it will start to flake if the paint is breathable.
To avoid these scenarios, it is best to wait for the plaster to dry completely before applying the mist coat.
In warm, sunny weather, it can take one week or more for the plaster to fully dry. In the moody British weather, it can take up to four weeks. You can leave the windows open and use a space heater to speed up things, but you should still allow the plaster to dry for at least one full week.
Mist Coat Drying Time
Mist coat generally dries faster than the undiluted paint. Depending on the emulsion you use, it can take anywhere from 30 minutes to two hours for it to become dry to touch. You should still allow it to cure for 24 hours before applying the finishing coats.
How To Tell If Mist Coat Worked
You can use a simple test to know if your mist coat worked. After it has dried for 24 hours, stick some masking tape on the wall. Peel it off and inspect the sticky side. If it comes off clean, the mist coat works, and you can apply the topcoat. Otherwise, you should apply a new layer of mist coat or use a primer specific for your substrate.
How do you dilute a mist coat?
You should always dilute the mist coat with clean water in a separate container. Measure the right amount of paint first, then top with water and stir thoroughly. The mist coat is properly prepared when the emulsion is smooth, and there is no water on top of it.
How long should you wait between mist coats?
One layer of mist coat is generally enough to prime the surface before applying the finishing coats, but you should wait for at least 24 hours. You should also wait for at least 12 hours if you want to apply two mist coats instead of one, then for an additional 24 hours before the painting.
Should you put PVA in a mist coat?
No. PVA is unsuitable for mist coats. The only suitable paints for mist coats are acrylic and latex emulsions, and you should only dilute them with water.
If you want to apply vinyl paint to mask imperfections, it is recommended to prepare the wall with a PVA primer.
Mist coats come as cheaper and easier-to-use alternatives to paint primers. There is a secret, though – diluting the paint properly. The best mist coat ratio varies from product to product and substrate to substrate. We hope this guide can help you figure out how to dilute your paint to avoid an unpleasant surprise.