Query Over Painting New Plaster - Is a Mist Coat Really Necessary?


Postby diydoctor6 » Wed Apr 06, 2016 3:48 pm

We recently had this feedback left about our project about Painting New Plaster. ( http://www.diydoctor.org.uk/projects/pa ... laster.htm )

"This is absolute rubbish, mist coat only came to the four front about 5 or 6 years ago it originally was to show any imperfection in the plaster so it could be corrected before the decorator finished the walls, before this happened were we called back to new plastered houses because the emulsion had flaked off, no and I should know I worked on new build properties for over 20 years. If you put your first coat of emulsion what ever colour with a small amount of water very small then a second coat of of same colour perfect job done no need for mist coat, and many decorators agree with me. Also were is this emulsion suppose to be sucked to I ask, plasterers only put a thin skim of plaster on to a plaster board........."

As you can imagine it has sparked a good healthy debate here at DIY Doctor! As a result we thought that we get the discussion out into the open...

Please join in!
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Postby thedoctor » Wed Apr 06, 2016 3:52 pm

Mist coats have been applied (and referred to) since Roman times when they realised just how porous the lime and pot-ash plasters were. They started to dilute the lime (lime wash) between coats of marble plaster and using diluted pigments to seal the walls before applying a new colour to the "Venetian" plasters. Which, as I'm sure you know, is a type of plaster built up from many coloured layers very thinly applied.

I'm afraid plaster sealers have been around since 1888 (when plasterboard was made from plaster layers inside wool paper) as clever businessmen recognised the porosity of the substrate and the lack of adhesion of the paints to the surface. The mist coat was adapted from lime wash to save money on the sealers. This is still the case today and while one or two plasterers may agree with your own theories I'm afraid you have contradicted yourself by saying that even you apply a small amount of water to the first coat of paint!!! This is a mist coat.

If you do not think plastered walls "suck" anything then please spray a plastered wall with a rose spray full of water and see how long it takes before any of the water runs down the wall. Do allow a bit of time for this as a square metre of skim plaster (whether on a base coat plaster or onto plasterboard) is capable of "sucking up" over 1 pint of liquid. Plasterboard is one of the most porous materials on earth!

Please take a look at all of the information about condensation and damp before you decide that plastered walls do not absorb (or suck) fluid.
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Postby diydoctor6 » Wed Apr 06, 2016 3:58 pm

Here's the reply:

"Thank you for getting back to me, I think we will just have to agree to disagree, first of all i was not around in 1888 so I don't know whether mist coats were used then or not but just because they were does not mean we should still be doing it today things have changed materials have changed, emulsion has changed, also when I said mist coats only came in 5 or 6 years ago I meant being used on new build properties today, Bellway, persimmons, Morris homes Redrow homes and many other house builders that I have worked for only introduced this practice then. The question that was being asked was do I need to put a mist coat on new plaster, the answer to that is no, some answers were you do have to put on a mist coat, and then a variety of reasons which were misleading, you say I contradicted myself because I said I put a small amount of water in my first coat of emulsion but I also put water in my second coat, this is to stop an orange peel affect from forming on the wall area being decorated so as to leave a much even and better finish just because you put water in emulsion does not mean it has all of a sudden becomes a mist coat, when does a final coat or a finish coat start only when it has no water in it, not true. Also I never said plaster was not porous obviously it is but not to the affect that if you don't put a mist coat on, the job is ruined and emulsion will start to flake off as some people suggested."

Some fair points here, so let's open up the debate....
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