How to Find the Best Mist Coat Ratio for Your Project

Finding the best mist coat ratio for your project is the difference between a quality surface with which paint is going to be able to key to and a surface that will leave you scratching your head as your top coats inexplicably peel off, leaving you with a mess on your walls and a whole lot of wasted time and money.

Fortunately, we’re using our years of experience (both good and bad) to share with you the best mist coat ratio for your project, along with a few extra mist coat tips to ensure your walls look the best they can be.

Mist Coat Ratio for New Plaster

The best mist coat ratio for new plaster will be according to the manufacturer’s instructions. This is because different emulsions will have different viscosities, therefore, will need to be watered down at different rates.

If you can’t find the suggested mist coat ratio, it’s a good idea to aim for 20% clean added water as this is what would typically be recommended.

Interesting to know

The reason you need to apply a watered-down coat of emulsion (commonly referred to as a mist coat) before painting on your top coats is that newly plastered surfaces are extremely porous and thus absorb liquids.

If you applied a coat of emulsion onto the bare plaster surface it will be too thick to absorb into the plaster and wouldn’t be able to adhere to the substrate properly, resulting in peeling or flaking.

The mist coat works brilliantly at soaking into the bare plaster and providing the perfect key for the top coats.

Extra tip

When mixing water with emulsion, it’s best to get your measurements spot on and then pour both into a mixing bucket. Unfortunately applying a mist coat is a laborious job as it’s best to use a brush to apply it due to the consistency. You can opt for a roller or a paint sprayer but expect having to clean up a total mess!

Mist Coat Ratio for Old Plaster

If you’ve moved into a property that has old plaster that has yet to receive a mist coat, it’s generally a good idea to avoid using a mist coat and instead use a primer that has been specifically manufactured for use on bare walls. Products such as Isomat Wall Primer would be perfect for this situation.

Depending on the condition of the old plaster, you might opt for a couple of coats of Zinsser Gardz as this is formulated to penetrate deep into old surfaces and provide a rock-solid substrate on which to paint.

If you’re removing old wallpaper and wondering if you need to mist-coat the plaster, I would personally advise against it. In this scenario, wash the walls, rub the walls down with fine aluminium oxide paper, and dust off the wall. You can now prime or apply your undercoat and top coats.

Mist Coat Ratio for Lime Plaster

Like common gypsum plaster, the perfect mist coat ratio for lime plaster is 20% clean water added. However, you really need to be careful with the type of emulsion you use for the mist coat and top coats. Lime plaster needs breathable paint and standard emulsion will block the wall’s ability to allow moisture in and out.

Fortunately, there are a few options available such as using limewash, however, the best emulsion I’ve used and would advise you to use too is Earthborne Claypaint. Whilst limewash is a traditional choice, it’s somewhat limited in colours whereas Earthborne Claypaint has a plethora of different styles to choose from whilst matching the breathable capabilities of limewash.

Top tip: make sure you go over the lime plaster with a damp cloth or water spray bottle just before applying the mist coat.

Do You Need A Mist Coat for Render or Masonry?

It’s not at all necessary to use a mist coat for render or masonry – you’ll only be wasting your time and money. Mist coats are only effective when used on plaster so I would advise against using them on render or masonry. Whilst applying a mist coat isn’t going to do any harm to your surfaces, it’s just a little bit pointless!

When preparing masonry for painting, the only things you MIGHT need to do is apply a stabilising solution if the surface is powdery or an alkali-resisting primer if the substrate is of an alkaline nature.

How to Tell if Mist Coat Ratio Worked

As long as you follow the manufacturer’s guidelines and measure the volume of water and paint to that ratio, your mist coat should be perfect (assuming you apply it properly of course).

However, if you want to see what a dried mist coat looks like, take a look at this example from a job I recently completed:

Example of a mist coat that has dried and is ready to be painted over.

Should You Apply 2 Mist Coats?

No, you shouldn’t apply 2 mist coats. If done properly, the first mist coat will absorb into the surface and provide you with a perfect substrate to paint.

Does the Mist Coat Need to be Perfect?

It’s not necessarily essential that your mist coat is perfect, assuming you don’t deviate too much from the suggested mist coat ratio. If you deviate 5-10% either way you shouldn’t cause yourself any problems.

However, if you water the emulsion down too much the walls are going to be too damp to paint and will take much longer to dry.

Likewise, if you don’t water the emulsion down enough then the mist coat won’t absorb into the plaster which can result in peeling or flaking paint.

Mist Coat Ratio As Recommended by Popular Brands

As mentioned above, different manufacturers create emulsions with different viscosities and therefore recommend their own mist coat ratio. In the table below, I’ve listed some of the most common brands and their recommended mist coats to save you the hassle!

BrandMist Coat Ratio
Dulux3 parts emulsion/1 part water
Johnstone’s Paint4 parts emulsion/1 part water
Crown3 parts emulsion/2 parts water
Tikkurila9 parts emulsion/1 part water
Farrow and Ball3 parts emulsion/1 part water
Leyland9 parts emulsion/1 part water
Little Greene4 parts emulsion/1 part water
Earthborne4 parts emulsion/1 part water

Additional Things to Bear in Mind

Whilst this comprehensive guide should give you all the information you need to get that perfect mist coat ratio, there are some other things to bear in mind before creating your mist coat.

Perhaps the most important factor in deciding the right mist coat ratio for your project is the state of your plaster. If your plaster is highly absorbent then typically you’ll need to add a little more clean water than what is suggested on the back of your tin of paint.