Dodgy mist coat advice

[opps]

As much as I love this site, I stumbled across the following

Mist Coat Ratio - Why it is important and what should it be.

A mist coat is the thinned first coat of paint used on new plaster or very porous surfaces. The main goal of this is to cover the surface and decrease its absorption capacity.

www.diynot.com

The article starts off well, explaining the importance of reducing the level of suction, recommending that readers follow the thinning instructions on the tin... and then...

A rather common recommendation is to take a 50/50 paint-to-water solution, but this is not without its disadvantages:

• The mix can be too thin, and the plaster will absorb it right away, which may lead to some patchiness.
• Getting good coverage might be a problem, and you may need to apply more coats.

You omitted to mention that if the mist coat is too thin when the full fat paint is rolled over the pseudo mist coat, subsequent coats of full fat might rip the previous coat off because the mist coat did little to nothing for the level of suction. I have lost count of the times that the builder has done the 50/50 before I turned up. I didn't notice until I applied the second full fat coat with a roller and sheets of paint started ripping off

The 50/50 recommendation should be sent to room 101... along with recommendations to apply thinned PVA to new plaster.

Oh, and recommendations to use dilute "contract matt" are wrong on many levels (not a recommendation on the linked page). The low latex paints are designed for plaster which is still slightly damp. They do zero for the level of suction.

Sorry, end of rant... and thank you for the site.

 

[Old Salt]

So what do we do instead?

 

[pete01]

Yes what do we do I've got some bare plaster I need to paint, saves me starting a thread.

 

[opps]

So what do we do instead?

As the first part of the article says, I would dilute the first coat according to the instructions on the tin of matt emulsion (note: not contract matt). I really don't understand why people on youTube think they know more about dilution rates than the company who makes the paint.

If the first coat is too thick, too much of the water gets sucked in to the plaster rather than evaporating off. Water-based paints cure through a process called coalescence- as the water evaporates off the molecules flatten down and bond together.

If the first coat is too thin, the second (full fat) will soak in too quickly and the coalescence process will "fail".

If you are painting the wall by brush, it is less of an issue. When you use a roller, the air is expelled from the roller as the paint is applied. If the previous coats have not cured properly the roller may well pull off the paint.