Painting New Plaster and How to Apply a Mist Coat

Painting new plaster is usually an essential step on the road to the completion of a home improvement project, but it’s not just as simple as firing the paint on the wall after the plastering has been done.

Simply doing this will lead to many problems such as damp issues and the paint flaking, cracking or peeling off the wall.

To prevent any such issues a mist coat, which is a form of priming coat made from mixing water with standard emulsion paint, is used on the wall first. This provides a safe and stable base for any further finishing top coats of paint.

Everything you need to know about mixing, applying and using a mist coat can be found below so read on.

What is a Mist Coat?

A mist coat is watered down emulsion paint mixed to a ratio of somewhere between 50/50 and 75/25 that is applied to new, bare plaster to seal and prime it to produce a stable base for any finishing top coats of paint.

New, freshly laid on plaster is extremely porous. What this means is that it can very easily suck up and absorb water, so when paint is applied to such a surface, the plaster sucks all the water out of it before it naturally dries preventing it from bonding correctly to the surface.

When this happens the coat of paint just sits on top the plastered surface and then either flakes off or can be easily peeled off.

Also as the paint is much thinner when mixed with water, it physically seeps into the plaster creating an unbreakable bond.

To ensure any top coats of paint applied to freshly plastered surfaces actually stay on the wall a mist coat is absolutely essential.

What is the Best Mist Coat Ratio?

There is a huge amount of conflicting advice on what the perfect mist coat ratio is, but to be honest, anything from 75% paint, 25% water (75/25) to 50% paint, 50% water (50/50) should be fine, depending on the thickness of the paint.

However when working on site we have found a combination of these ratio’s to work best by applying the first mist coat at 50/50 and then a second coat at 75/25. We have found this method consistently gives the best and most stable base for top coat paint.

Standard Mist Coat Mix Ratio

• 4 parts paint
• 1 part water

Mist Coat Mix Ratio for Thick Paints

• 1 part paint
• 1 part water

The mix itself should be fairly watery and thin as the water content has to be enough to both satisfy the absorbent nature of the new plaster and also allow the paint to dry properly for it to create a good bond with the surface.

Despite the above advice we’ve given on use on site, some emulsion paints are thicker than others. If you have a particularly thick paint then a 50/50 mix should be used, however if it’s a standard consistency then 75/25 can be used.

If in any doubt there should be manufacturers instructions on the paint canister itself indicating the correct mix ratio for that particular paint.

What Kind of Paint do You Use on a Mist Coat?

You should use a general purpose water-based matt emulsion paint to make a mist coat. As you should only use water-based paint for a mist coat for fresh plaster due to it’s porous nature, this emulsion ticks all the boxes.

Although technically you could use gloss emulsion, eggshell and other emulsion paints, these will generally require more water than standard matt emulsion, so it’s best to stick with this.

Do not be tempted to use vinyl or silk paints! As these types of paint dry, they form a “skin” that sits on top of the bare plaster and does not bond with it meaning that it can easy flake or peel off.

In terms of the type of emulsion paint to use, a cheap contract matt will do fine, no need to go for the expensive stuff, although some professional decorators would argue otherwise.

Although we have stated using white matt emulsion, you can use other colours. We only suggest this as a light neutral colour will provide a good base for your finishing topcoat paint.

Some more modern and specialist paints recommend you should only use a watered down coat of the same product you’re using for your top coat or even a specific mist coat or priming product from that manufacturer. If this is the case always follow the manufacturers instructions.

How do You Make a Mist Coat?

The best way to make a mist coat is to use a measuring jug to measure the correct paint and water quantities and pour them both into a paint kettle or mixing jug and then use a clean stick or drill and mixing paddle to thoroughly mix them.

What ever mixing method you use make, sure you mix the components up thoroughly! A good few minutes should be adequate.

Once mixed, apply using a brush to a small area of bare plaster. If the plaster does not seem to be sucking up the liquid quickly, more water can be added to a maximum of 1 part water to 1 part paint.

If you are sure that the mix is correct, you can get it on the wall.

What to Use to Paint a Mist Coat on a Wall?

When it comes to actually painting new plaster, you have really 2 options: a paint brush or a roller.

A large paint brush (around 4 inches) is really the best option as it gets a fair amount on the wall with the minimum of splash. You can use a roller and this is certainly the quickest method, but it’s also the messiest by far as it flicks paint everywhere!

How Many Mist Coats Should You Apply?

In most cases applying one mist coat to your new plaster should be enough to give a stable base for a top coat. However applying 2 mist coats, the first mixed 50/50 and the second 75/25 will pretty much guarantee no issues. Allow 24 hours between each coat for drying.

As long as you have allowed your plaster to fully dry according to the information above, applying just one coat mixed at around 75/25 should be enough and will create a solid enough base for any finishing top coats.

However the belt and braces approach of applying 2 mist coats is the best way to go as this way you can be certain that you won’t experience any flaking or peeling paint issues.

When all plastered surfaces are sealed and dry, you can continue to paint as normal and apply any top finishing coats.

Do You Have to Mix Your Own Mist Coat?

It’s worth mentioning that today there are specialist paints available, designed specifically to be applied to bare plaster without the need for making and applying a mist coat.

Here at DIY Doctor, we have never used these products before as we prefer to stick with the traditional mist coat method as we know this works, but we do know many that have tried them. All we will really say on them is we have heard mixed reviews; some that have seen great results and others that haven’t!

How Long Should I Wait before Painting New Plaster?

Before painting new plaster you must wait until the plaster is completely bone dry! In terms of time, in a standard home with central heating or during the warner months this will be around 3 weeks.

However there is no definitive answer to this question as all walls and ceilings will dry out at different rates depending on many factors such as humidity, air temperature, the type of property, the type of plaster, how many coats of plaster there are, whether the central heating is on, the time of year and many other factors.

In extreme situations, a fully plastered wall, i.e. two or even three coats of plaster, can take months to dry out fully!

So, how do you really know when it’s dried and it’s safe to paint new plaster? The best gauge for this is the colour of your new plaster. The first image below shows a newly plastered wall. As you can see, the plaster is dark in colour as it has only just been laid on.

The second image below that shows the same area after about a week. As you can see some areas are much lighter, indicating that they have dried out, but there are still quite a few dark patches that they are still damp and have a fair amount of drying to do.

The third image below shows the plaster after about 2 and half weeks. As you can see it has now dried out completely and is a uniform light colour across the entire area, giving a good indication that it is now dry and safe to paint.

The type of plaster you use will also affect its drying time as there are quite a few different types of plaster (information on these can be found in our plastering and skimming and types of plaster project) and each has it’s own approximate drying time.

However, any approximate drying times will also depend on what the plaster has been applied to e.g. direct to plasterboard or on to a bonding coat over brickwork, concrete blocks etc.

What’s the Best Way of Painting New Plaster?

Step 1 – Prepare Plaster Surfaces

Before any mist coat is painted on the surface needs to be prepared. Go over the surface with a scraper and scrape off any lumps, try not to sand as this creates a lot of dust that can get into the mist coat.

However in some cases you may not have any other choice other than to sand, in which case use paper somewhere between 120 and 240 grit to gently rub any marks out.

Don’t rub too hard or too much in one place as you will end up causing more damage by creating further dips or indeed polish the plaster to a point that the paint will no longer adhere to it fully. Using a flat rubbing block is a good idea as this will help ensure the paper stays flat to the surface and that you don’t create any dips.

Any imperfections such as scrapes, trowel marks and other depressions should be filled with a suitable filler, left to dry and then sanded flat.

Painting on a mist coat is a very messy job so all floors, skirting boards, windows and anything else should be covered with dust sheets and taped down, and ensure you wear old clothes and gloves.

Step 2 – Mix up a Mist Coat

For this example we are going to use our 2-coat mist coat method mentioned above so first measure out enough paint for the size of room your painting and pour it into a paint kettle and then add an equal amount of water for a 50/50 mix.

Use a clean stick or mixing paddle to thoroughly mix the paint mixture in the kettle.

Step 3 – Apply First Mist Coat to Plaster

Using a 4 inch paint brush or paint roller and starting at the corner of one wall, paint the mist coat on the wall. Make sure each coat is even and there are no runs.

Once the first coat is on the wall, leave it for a full 24 hours to dry out.

Step 4 – Apply Second Mist Coat

Mix up your second mist coat to our suggested ration of 75/25. Measure out the components using a measuring jug, add them to a paint kettle and then mix thoroughly.

Paint the second mist coat on to all surfaces using your preferred method and again, leave for a full 24 hours to fully dry out.

Once you have confirmed that your second coat is dry by passing your hand over it, you can then start painting on your top coat.

What Happens if You Don’t Apply a Mist Coat?

If you don’t apply a mist coat or correctly seal new plaster, the simple answer is that what ever you put on to it e.g. paint, it will simply sit on the surface and eventually flake or peel off.

For a coating such as paint to bond to a surface correctly so that it stays on there, it must be allowed to dry out correctly and also bond to that surface so that it stays there.

As new plaster is extremely porous applying a mist coat is essential as the additional water in the mist coat is sucked up by the plaster while the remaining water in the paint allows it to dry and form a solid bond.

Do You Really Have to Wait That Long for New Plaster to Dry?

The short answer to this is yes! Wait until it’s totally dry and the same colour as in the third image above.

The reason for not painting new plaster before it’s completely dry is that most paints will simply form an air tight skin over the wall.

The moisture from the new plaster is then trapped behind this skin and cannot evaporate off. The damp then either retreats back into the wall where it develops mould growth or reacts with the salts in the wall to become Efflorescence. Either way you have a problem on your hands that is incredibly difficult and potentially costly to deal with.

Can You Make New Plaster Dry Quicker?

In short, no! To get the best possible finish results, fresh plaster should be left to dry out naturally.

As with many materials and finishes in the building trade that require drying time, if you try to rush this drying or curing process then in most cases the “bond” between surfaces that is created during this process is weakened and in this case your new plaster won’t stick fully to the surface beneath it.

Additionally, rapidly drying freshly laid plaster can also shrink it, causing cracks to appear in the surface.

Despite the above, there are things that you can do to aid the natural drying process e.g. ventilation.

If possible, open all windows and doors to help create as much ventilation and through-draft as you can as this will help with the natural drying of your new plaster.

What if You’re in a Real Hurry to Paint New Plaster?

If you are in a hurry to paint your wall because Grandma is coming to stay, there are paints on the market called Microporous Paints.

These paints allow the surface of the plaster or indeed artex to carry on breathing and evaporating while they are drying out. They can be found in a lot of DIY stores and decorators merchants.

Microporous paints are generally a lot thinner than ordinary water or solvent based paints and in a lot of cases, once the wall is dry, it is recommended that you over-paint them with emulsion because of this, but only when the plaster is totally dry.

If you are going down this route, please read the manufacturer’s instructions on the container for information regarding thinning down and applying the first coat (mist coat) onto new plaster.

Although this method is a solution if you are in a real rush, we would always recommend waiting the required drying time before painting new plaster. In the majority of cases, you will never get a finish you are totally happy with when rushing, regardless of whether you use specialist products or not.

In summary, there is no substitute for leaving lots of time to allow your new plaster to dry fully before painting it. Wherever possible err of the side of caution and leave it as long as possible before painting new plaster, but we also recognise sometimes time is not on your side and you just have to get it painted, but be aware of the risks and issues that this could bring.