Mist Coat Ratio: How To Paint Fresh Plaster

Mist coat is one of the easiest to use, most readily available, and most affordable primers for paint jobs. Therefore, it is a popular choice for both major and minor painting projects. However, its effectiveness depends on the mix ratio, which is why this article exists.

Most times, the “optimal mist coat ratio” is the popular choice of 80:20 or 70:30. However, that is not always the case. A good Mist coat ratio is determined by paint quality and the substrate’s porousness. So, keep reading to know the right mist coat ratio for some common substrates and paint manufacturers.

What Is the Optimal Mist Coat Ratio?

The optimal mist coat ratio is 80:20 (80% paint to 20% water) for new plaster and 50:50 (50% paint to 50% water) for old plaster. By new plaster, we mean plaster that has never been painted. Old or existing is one that has been painted before. This is the ideal mist coat ratio for other substrates, too.

However, a mist coat ratio should depend on the paint quality and the substrate’s porousness. Substrate porousness is the most important factor, but paint quality also plays a huge role. That’s why this article has a section dedicated to mist coat ratio recommendations by some of the most popular paint manufacturers in the UK.

Here are the optimal mist coat ratios for common substrates in the UK.

Substrate	Optimal mist coat ratio
New plaster	80:20
Old plaster	50:50
New masonry	80:20
Old masonry	50:50
New render	80:20
Old render	50:50

Manufacturer Recommended Mist Coat Ratios

Once again, paint quality determines the ideal mist coat ratio. So, it’s not always appropriate to use the same mist coat ratio for every paint product is not always appropriate. Even better, always go with the mist coat ratio recommended by the manufacturer. Only use the 80:20 ratio if the manufacturer doesn’t include recommendations.

Paint Manufacturer	Recommended mist coat ratio for new plaster
Crown	85:15
Dulux	75:25
Leyland	90:10

Why Do You Need Mist Coat or Any Other Type of Paint Primer?

Primers are the secret weapons of professional paint jobs. For paint to stick, it has to bond to the surface being painted. This happens through absorption. The substrate (plaster, render or masonry) absorbs the paint.

Moisture is the bonding agent between substrate and paint. So, the absorption rate depends on the moisture level of the paint. The moisture level of regular paint/top coat is too low to adhere effectively to new plaster. It’s too concentrated for plaster to absorb. The resulting paint job won’t look good. It will also soon start to peel, flake or blister.

That is why you need a primer. You may also hear people refer to primers as sealants or undercoats. A primer is less concentrated and more adhesive than the top coat. It is a sealant for the surface (plaster or otherwise) and a base that makes it easier for the top coat to stick, giving you a beautiful, pro-looking result.

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 fact that it’s the same paint as the top quote also prevents further issues.

What Can Affect the Mist Coat Ratio?

Here are the factors that affect the mist coat ratio.

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 substrates are plaster, masonry and render. Porousness varies between these materials, but age is the most important cause of variation. New walls need a less diluted mist coat compared to their older counterparts. However, if an older wall is stripped, it needs a less diluted, thicker mist coat.

Painting Method and the Amount of Mist Coat Layers You Intend to Use

These two factors aren’t as important because they are based on individual preferences. However, we should still talk about them. Someone using 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.

How to Make Mist Coat

Follow these steps to make a mist coat for fresh plaster. It’s a pretty easy process.

• Step 1: Set aside a large bucket, paint tin or a substantial container.
• Step 2: Add the required quantity of paint.
• Step 3: Top this with the required quantity of clean water to dilute the paint. Remember to use the mist coat ratio we recommended for fresh plaster.
• Step 4: Stir thoroughly until well-mixed and smooth.

You can use a bowl for measurements. For mixing, use a stick, drill or mixing paddle.

How to Use Mist Coat When Painting Fresh Plaster 

Application is just as important as the mist coat ratio. Follow the following steps to use the mist coat properly.

Step 1: Let the Plaster Dry

It is crucial to allow newly plastered walls to dry before painting. Drying can take one to six weeks, depending on weather and ventilation. Here are three indicators that plaster is dry: 

• Dry to touch
• Lack of damp patches
• Light, creamy pink colour: wet plaster is brownish or dry grey

Step 2: Mix the Mist Coat

Follow the guide we discussed earlier. Remember to use the recommended mist coat ratio.

Step 3: Apply Mist Coat and Let It Dry

Use a large paintbrush or roller to apply the first coat to the drywall. There is going to be some splattering, particularly so with a roller. So you should protect yourself with clothing overalls and place dust sheets or newspapers around the entire area.

How Long Does It Take for Mist Coat to Dry?

Mist coat usually dries within 1 to 2 hours. Wait for it to cure before adding the top coat. Curing allows the substrate and mist coat to bond properly. The top coat will also adhere better as a result.

How Do I Tell if Mist Coat Has Worked?

There is a test you can use. However, you must first wait for the mist coat to dry and cure. After curing, 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. It’s time to add the top coat.

How Many Mist Coat Layers Should I Use?

Even when painting a newly plastered wall, which is usually more porous, you rarely need more than the first mist coat layer. So, if some parts of the plaster 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 test whether the current mist coat worked before adding another or a third layer.

When Can I Paint A New Plaster?

You can start painting new plaster as soon as it is dry. Drying takes about 1 to 4 weeks, depending on the weather. You can open windows and doors or use central heating to speed up the process. Anything that can improve ventilation and air temperature will shorten drying time.

Whatever happens, never paint wet plaster. The paint will prevent the plaster from drying. The paint itself won’t adhere properly. It will crack, swell, peel or flake. You can use breathable paint to prevent this, but it doesn’t always work. If you want the best possible result, wait for the plaster to dry.

Plastering and painting require a lot of waiting and patience. There is no other option because the consequences of painting wet plaster or not using a primer are nearly impossible to fix. However, breathable paint is the best option for people who can’t wait. It is more porous than emulsion, allowing the plaster to keep drying after being painted.

Conclusion

In most cases, the ideal mist coat ratio is 80:20 (80% paint to 20% water) for new plaster and 50:50 (50% paint to 50% water) for old plaster. However, this is still just our recommendation. The paint manufacturer might suggest a different mist coat ratio. Your judgement and experiences may also suggest something else.

Listen to the manufacturer. They are the expert on their product. However, if you have to choose a mist coat ratio yourself, always consider paint quality and the porousness of the substrate. They are the only factors that matter.

Frequently Asked Questions (FAQs)

How Long Should You Wait Between Mist Coats?

Wait for 12 hours. That is enough time for the current coat to dry.

Do You Need Mist Coat for Render?

Yes, renders need a mist coat. Masonry does, too. The differences between these substrates are not enough to change the rules of mixing and applying mist coats. So, our recommended mist coat ratios apply to render and masonry.

How Do You 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 (of paint to water) and slowly work your way to 1:1.

However, we recommend not going beyond 70:30. You can use a slight touch test to see if you have a good mix ratio. However, here is a simple yet effective test. Use a brush to apply some mist coat and monitor how long it takes plaster to absorb it. 

What Are the Best Types of Paint for Making Mist Coat?

The best paints for mist coats 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.

Should You Use PVA as a Mist Coat?

No, you shouldn’t use PVA as a mist coat. It’s best to purchase PVA primers instead. A mist coat is supposed to be porous and diluted. PVA does not have these qualities. Also, it is waterproof, and most types of PVA aren’t meant to be diluted. Therefore, PVA will impair adhesiveness and moisture movement, two things that mist coat is intended to aid.