Mist Coat Ratio: How To Paint Fresh Plaster!

Mist coat is arguably the most important element to putting the finishing touch, the paint, on any building work. It is usually the biggest difference between an amateur and a professional paint job. This could be argued to be not completely correct, but there is no doubt how crucial the primer is when painting walls.

The primer which is known as sealant or undercoat is the key to painting. If a painted wall (especially recently painted) suffers from paint peeling, flaking, blistering and other blemishes, it is probably because a primer has not been used at all or has not been used properly.

Out of all available primers, mist coating is probably the most common, affordable and easiest to use, especially on minor and major home improvement projects. However, all of this depends on getting the mix ratio and application right.

What is a Mist Coat and What is It For?

For paint to stick, the surface that it is applied to has to absorb it and the two have to bond with each other. This bond/adhesion is aided by the paint’s moisture content. The more moisture it has, the easier it is to be absorbed by the surface or environment (commonly known as evaporation).

Let’s focus only on absorption by surface for now. Paint that is too thick will not bond with or stick to the surface because moisture (the bonding element) is insufficient. When this occurs, blemishes are likely to appear as mentioned earlier.

That is why you need a primer that is less concentrated and more adhesive than the top coat. It serves as a sealant for the surface (plaster or otherwise) and a base that makes it easier for the top coat to stick which will give you a beautiful, pro-looking result.

A mist coat is the easiest primer to find and use because it is just diluted paint. Formed from paint and water, the higher moisture content of the mist coat allows it to improve adhesiveness for both itself and the top coat.

The best paints to use for mist coat are water-based products such as latex and acrylic emulsion. Vinyl silk and oil-based paint are not well-suited for this role. It is also best to use a neutral, lighter or similar colour as the desired top coat.

What is the Best Ratio for Mist Coat?

Surface	Mist coat ratio
Newly plastered wall	4:1 (80/20)
Old wall	1:1 (50/50)

• New surface- 4:1
• Old surface – 1:1

There is no perfect mist coat ratio. Individuals will have their own preference of what makes a ‘good mist coat ratio’ based on what works for them. Our recommendations are based on past experiences. So here is a tip; check the manufacturer’s instructions for recommended mist coat ratios when buying paint.

However, you may decide to go with a more diluted or thicker mix ratio. This decision depends on you, the surface (substrate) and the original quality of the paint product. There are no rules except to find a balance between being too watery and too thick.

But if it becomes too diluted, the mist coat will evaporate before the surface can absorb it and if it is too concentrated, it will not be absorbed.

What Can Affect this Ratio?

There are several factors that can affect mist coat ratios: 

The quality of the paint product: Higher quality products are usually thicker in consistency, so you can use more water when diluting them.

The porousness of the substrate: The three most common substrate types are plaster, masonry and render. Porousness varies between these different materials, but age is the most important cause of variation. New walls need a less diluted mist coat that is thicker in consistency than their older counterparts. However, if an older wall is stripped, it will also need a less diluted, thicker mist coat. There are two more variables to consider which are the painting method and the amount of mist coat layers you want to use.

These two variables is down to the individual preference. For example, someone who wants to paint with a roller will require a less watery mist coat to minimise paint splatter while the opposite is true for someone who wants to use multiple layers of mist coat.

Here’s What You Need to Do Step By Step:

After mixing, the application is also an important aspect of getting the desired result. So follow the following steps to use the mist coat properly:

1. Let the plaster dry

Drying can take one to six weeks, depending on weather and ventilation. The first thing you should know is it is crucial to allow newly plastered walls to dry before painting. However, you can’t tell if the plaster is completely dry by simply just looking at it.

Three indicators you can rely on include touch, lack of damp patches and colour. Wet plaster is a brownish or dry grey colour, while dry plaster is light, creamy pink. So take this into consideration so that you have a clear colour all over.

2. Mix a mist coat

It is pretty easy to mix a mist coat. Firstly, set aside a large bucket, paint tin or a substantial container then add the required paint quantity. Top this with the required quantity of clean water to dilute the paint then stir thoroughly until well-mixed and smooth. It is better to use something like a bowl for the measurements. For mixing, use a stick, drill or mixing paddle.

3. Apply the mist coat and let it dry

Use a large paintbrush or roller to apply the first coat to the fully dried wall. There is going to be some splattering, particularly so with a roller. So you might want to protect yourself with clothing overalls and place dust sheets or newspaper around the entire area.  

How Long Should I Wait Before Painting fresh plaster?

Make sure the plaster is completely dry before you start painting. The drying time can vary from one week to six. It all depends on weather and ventilation. For example, it will dry faster in the summer than during winter or spring, but it’s best to be patient.

Painting on wet plaster is just as bad as painting on bare plaster without using a seal or primer. As the paint dries, it forms a wall of emulsion that doesn’t prevent the plaster from drying. As a result, the affected plaster becomes prone to efflorescence and mould, alongside other blemishes that happen when the paint is unable to bond with a surface. Even though the mist coat is still breathable, it is also guilty of this same crime, trapping moisture.

How Many Mist Coat Layers Should I Use?

Even when painting a newly plastered wall, which is usually more porous, you will rarely ever need more than the first mist coat layer. So, if you think some parts of the plaster still need coating, it is more efficient to treat the affected areas.

That will save you both money and time. However, there are times when you may need a second mist coat layer. It’s best to use your judgement in this case. You can run a test to see if the current mist coat worked before adding another or a third layer.

FAQ:

When Can I Paint New Plaster?

You can start painting new plaster as soon as it is completely dry. Although drying can take a while, you can leave the windows and doors open or use central heating to help the process. Anything that can improve ventilation and air temperature will shorten drying time.

How Do I Achieve Optimal Mist Coat Ratio?

The secret to a good mist coat ratio is to find a balance between too watery and too concentrated. Finding the optimal ratio on your own takes experience and good judgement. To experiment, start with a 9:1 ratio paint to water ratio and slowly work your way into 1:1.

However, we recommend not going beyond 70:30. You can use a sight or touch test to see if you have a good mix ratio. However, the simple yet effective test of using a brush to apply some mist coat and seeing how long it takes plaster to absorb it is a successful way.

How Do I Tell If Mist Coat Has Worked?

There is a test you can do, but you have to wait for the coat to dry before trying. This (drying) usually takes 1 to 2 hours. However, it usually takes over 24 hours for the coat to fully dry and cure. After curing, it will be in the best condition possible to help the top coat bond with and stick to the plaster.

Follow these steps to test the mist coat. Attach a masking tape to the painted wall, wait a few seconds then peel. If the tape comes off clean, it means your mist coating worked.

How Long Should You Wait Between Mist Coats?

After 12 hours, this is more than enough time for the current coat to dry so that the next one can also work effectively.

Is It Essential To Wait That Long?

Yes, if you want the best possible result it’s best to wait. While it’s time-consuming, it’s best for the process to be patient. The consequences of painting wet plaster or not using a primer are nearly impossible to fix. Most times, the only solution is to start over or accept bad quality work.

However, there are options if you can’t wait weeks for the plaster to dry. You can use breathable paints instead of regular matt emulsion for the mist coat. The former is more porous, allowing the plaster to keep drying after being painted.

Do You Need Mist Coat for Render?

Yes. Render also needs a mist coat, as does masonry. The differences between these substrates are not enough to change the rules of mixing and applying mist coats. So our recommendation re mist coat also applies to render and masonry paint jobs.  

Should You Use PVA as Mist Coat?

No, it is best to not use PVA as a mist coat. PVA is more appropriate for covering imperfections such as holes, cracks, and shifts in walls. A mist coat is supposed to be porous and diluted which are qualities PVA does not have.

Also, it is waterproof and most types of PVA are not meant to be diluted. It will impair adhesiveness and moisture movement, two things that mist coat is intended to aid. So it’s best to purchase PVA primers instead when you want to use PVA paint.