Number of mist coats recommended for the newly skimmed walls.

12 Jul 2011
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United Kingdom
I have just put on a mist cost on the freshly skimmed wall. How many mist coats does it normally need before putting on the top coat? Would 1x coat be enough, or 2-3 coats?
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Many people just do one. I prefer two. On rendered walls, concrete, or bare blocks, it may take more.

The first thinned coat will soak in and disappear. It is dry almost at once, so you can walk back to the start and do another. You will know you have killed the suction when it no longer sucks the moisture out of your brush.

It is very cheap, very easy, and very quick, so for DIY work, not really a problem. If you were on a contract to do hundreds of metres, it would be different.
I will put another 1-2 mist coats before applying top coats. I have much waterbased matt. paint for mist coats available from the old used paints. Thanks.
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The wall is the extention of the kitchen space, so it had not been sanded down after the skimming to avoid dust coming into the kitchen area. So it is kinda rough with lots of bumps.
You can sand walls using sanders connected to dust extractors (read: vacuum cleaners).

If you don't want to go to the expense of buying an electric sander, you can get a hand sander that connects to the vacuum hose.

The Mirka hand sanders are pretty good, I have two (small and larger).

Important- If you are using a domestic vacuum cleaner, empty the bag (very) frequently. Plaster dust clogs the pores in the bags very quickly. If you have a Dyson, the dust will ruin it.

When I paint new plaster, I sand it first and apply one mist coat, I then do a wet on wet dilute coat (a total of 1.5 coats). The main reason I do that is because I hate the idea of the client walking in and thinking "OMG the painting looks awful". A wet on wet looks like a full fat coat over a watered down coat.

I would then apply two full fat coats after (but if you are using an inferior quality paint, you may need 3 full fat coats).
Maybe just leave it at 1 coat but put 2-3 top coats? Would it be better option?

The purpose of the mist coat is to deal with the porosity of the plaster (known as suction).

Waterbased paints cure through a process known as coalescence. As the water evaporates off, the molecules shrink down to the substrate and bond together. If you were to apply full fat paint over new plaster, much of the water in the emulsion would soak in to the wall and hamper the coalescence process. The paint effectively dries on the surface and if you were to roll another coat over it later, there is the possibility that the water in the second coat will pull the first coat off. I have encountered that many times, and when I do, I often have to spray the whole wall with water and scrape the old paint off.

By diluting the mist coat, you are increasing the amount of water in the paint. Some will be immediately sucked in to the plaster but there will be enough left to help with the coalescence.

Please ignore anyone on the internet that says "add X amount of water". The manufacturer of the paint will print the recommended amount on the tin. Dulux Trade is often 10% but other brands may be higher.

As an aside, for small jobs I use Leyland Trade acrylic primer over new plaster. It can be used neat.
I have already put on 1x mist coat, which was Dulux (with 10% water dilluted). It was actually rough measure - could have been 15 - 20% water, but it seems applied OK. I can see a bit of bare patches on some parts, but mostly looks ok solidly dried.

I don't think I can do sanding at this point because that would take out the mist coat which is already on the wall, unless I paint another mist coat after sanding. I have a hand sander with the sanding paper and also shop vac which can link to the electric sander - and have used them in tandem before.

But maybe I can go ahead with 2x fat top coats and finish it off? That will be tomorrow.
Mist coats sand back nicely. When you sand back any high spots, just brush/roll another mist coat over those areas. You should be looking at 80-120 grit paper, followed by a finer grade to eliminate any scratch marks.

Apropos, rates of dilution- yeah, I too guestimate. It isn't an exact science. If 10% is recommended and you use 15-20%, you will normally get away with it. I get annoyed when people recommend using a 50/50 mix. Mind you, that isn't as bad as the people that recommend using PVA as a primer....
PVA dried on the wall will not stick any paint at all. That is shocking people advicing that. :/
Thank you for great advice. Yeah, I will try sanding a bit and do more mist coats. It is no hurry finishing this job luckily.

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