I’ve been a painter and decorator for the best part of 20 years and often blog for the Decorators Forum UK. I thought I’d put pen to paper and explain a little bit about the importance of a mist coat when decorating. I hope it helps.
What is a Mist Coat in Decorating?
A mist coat is the first coat of paint on bare plaster. The purpose of a mist coat is to prime and seal a wall or ceiling to create the perfect base for additional coats of emulsion. If you miss the mist coat (try saying that after a few beers), you run the risk of adhesion issues and paint failures.
You should not sand bare plaster before you apply your mist coat as you run the risk leaving a thin layer of dust which will also cause adhesion issues. Just scrape off any snots (drips or lumps of plaster) and apply your mist coat. You can sand afterwards if needed.
What is the Best Paint to Use as a Mist Coat?
A lot of decorators use a cheap contract matt as a mist coat. However, some manufacturers do not recommend using contract matt anymore, although it is still effective.
The best advice I can give you is to check the manufacturer’s instructions on the product you intend to use as your finish coat.
Most vinyl or durable matt emulsion products now advise a diluted coat of the same product. Some brands of emulsion, such as Tikkurila or Farrow and Ball require a specific primer made by the same brand. Do not deviate from these instructions as you will not be covered in the event of product failure.
What is the Ratio for a Mist Coat?
You would normally add a substantial amount of water to an emulsion if you intend to use it as a mist coat. The ratio of water to paint changes slightly, depending on which paint you use. If you are using a cheap contract matt, or a paint which is specifically designed for use on bear plaster, then 10% – 20% water is absolutely fine (1 or 2 parts water to 10 parts paint).
For most products, I’d suggest adding more water than this. Vinyl matt for example should be around 25% water. Some acrylic eggshell or soft sheen emulsions can be used as a mist coat, and these need to be diluted even further. Always check the manufacturer’s instructions but add a little bit more water than they suggest. I have seen many issues caused by not adding enough water to a mist coat, but not one issue from adding too much. It’s better to be safe than sorry!!
How Should a Mist Coat Look?
It really does not matter how a mist coat looks when it’s on bear plaster. You don’t need it to cover, all you need to do is make sure it adheres to the plaster and leaves a good base for additional coats of emulsion.
Can you use vinyl Matt as a mist coat?
Yes, diluted vinyl matt makes a great mist coat. Just be sure the plaster is completely dry first, as vinyl matt will not allow the substrate to breathe as well as contract matt does.
What is the Best Way to Apply Emulsion to a Wall or Ceiling?
I know this isn’t directly related to applying a mist coat, but I thought I’d include it. I actually spray most of my paint now, however most people will use a brush and roller. Simply “cut in” around the edge of your ceiling or wall with a brush, then roll the middle using a roller. Dead easy!! Always allow a mist coat to dry fully before carrying out any additional prep to your walls or ceiling.
What Happens if You Forget to Apply a Mist Coat
If you forget to apply a mist coat, or you apply an unsuitable paint as a mist coat, then you will almost certainly experience problems. These problems will not necessarily rear their head straight away, although they might.
The most common problem caused by missing a mist coat is pealing paint, but sometimes it just bubbles. You may see it as you finish your decorating, or it might happen next time you decorate the room.
Applying a mist coat is an important, but simple part of decorating a room. Always choose the emulsion you intend to use as a finish coat and check the instructions on the tin. Most manufacturers will tell you what product you need to use for a successful mist coat.
A lot of professional decorators still use contract matt, and it works well in most circumstances.
Mist Coat When Decorating – Questions and Answers – by Mike Gregory