A mist coat is a great idea when painting any porous surface. In case you were unsure a mist coat is simply a watered-down first coat of paint. The mist coat is essentially a primer/sealer. Porous surfaces such as new plaster will soak up paint which is why we use a mist coat. The mist coat saves us paint. The watered-down first coat then acts as a partial sealer once dried, preventing second coats from being sucked into the surface. It also helps prevent the paint from drying out too fast due to the moisture being sucked out by the porous surface. This is all great to know, but you are obviously wondering how much you should water down your paint, in other words, what is the perfect mist coat ratio?
What is the perfect mist coat ratio?
For me, the best ratio was 2:1, 2 parts paint to 1 part water. This was a good medium of allowing the paint to go much further and thus save money while not being too thin to paint well. Unless you were to measure it out individually then this will obviously just be a rough estimate. There really is no need to be bang on here, you just need it to be thick enough to cover while watered down enough that it is saving you paint and doesn’t dry out too quick!
It is also worth mentioning that I used high-quality paint here which was reasonably thick to start with, so with cheaper paints go for lower water to paint ratio.
What can affect this ratio?
There are lots and lots of different factors that can change what ratio to use. One obvious factor is how thick the paint is to begin with. Some cheaper paints are really thin to begin with, so don’t require as much water as a higher quality and thicker paint would. The surface you are painting will also change what the perfect ratio is. More porous surfaces will suck up more moisture and therefore a more wet mixture may be better.
then there is also your painting method to consider, if painting with a roller you will want a thicker mixture. Anyone who has painted emulsion with a roller will already be well aware of the splatter it creates, now imagine if that paint was heavily watered down and incredibly thin. Yeah, one big, splattery mess.
Testing the ratio
So with all this in mind, I decided to do some testing to try and find a good ratio. I wanted to know how well each different ratio covered the surface. I then wanted to see what effect this had on the second coat, after all, if the aim is to save paint then you don’t want a mist coat that is so thin that multiple secondary coats are required.
For the test, I painted a new plasterboard wall that had never been painted before. While not as good of a test as painting a newly plastered wall this is all I had, so I ran with it.
I split it up into four different sections and then painted these with different ratios of mist coat. The top would have the original paint, not changed at all. The next section would have a 2:1 ratio where it is two parts water to one part paint, the next a 1:2 ratio (1 part paint to 2 parts water) and then finally a 1:4 ratio (1 part paint to 4 parts water).
1:4 Ratio Mix
Let’s get started with the most watery mix, the 1:4. 1 part paint to 4 parts water
Here is the 1:4 mixture in the tub. I can already tell this is waaaaay too watered down, it is pretty much just a slightly white coloured water at this point.
This is it up on the plasterboard. This paint mix is so watery it is incredibly tough to paint it. It just runs down the wall, it is almost as if I’m trying to paint a wall with water…
1:2 Ratio Mix
You can see here that the paint is still really watery, I think this will still be a bit too much water.
Painted on the plasterboard you can see that the 1:2 mix has actually covered quite well. You can see the stark difference between coverage with this mixture over the 1:4 ratio. However this was still too thin to paint with, runs are a serious problem with this ratio. And because of how wet the paint is the runs appear and disappear quickly, much quicker than you can react to catch them. If you tried painting a room with this mixture it would be very messy!
2:1 Ratio Mix
The 2:1 mixture in my paint tub. You can still see that this is a very watery paint compared to a normal emulsion but a lot thicker than the other ratios I have tried so far.
I think the above image shows the difference between these mist coats well. It is not massive but there is a clear difference between how well each different ratio has covered the plasterboard.
I also found the 2:1 mixture quite easy to paint with. it went on more like regular paint whereas the other two ratios felt more like I was putting a wash on rather than painting.
No Mist Coat
Finally, at the very top of the board, I have painted a section using no mist coat at all. This is here to illustrate the difference in coverage between the regular paint and the mist coats.
So now I have the mist coats down it is time to apply a regular second coat over the top. This will just be the regular paint straight out of the tin.
This will let us know whether the mist coat has covered well enough that we don’t need tons of coats of regular paint over the top. After all, if we need three extra coats to cover the wall because of how thin the mist coat was then has it really saved us any paint?
So here we can see the second coat on the plasterboard. There isn’t much between them, to be honest, but this effect would be greater on a freshly plastered wall.
The most watery mist coat ratio at the bottom has barely covered the plasterboard. Once dry it is just a white wash, this was way too much water.
The next two are much closer, but the 2:1 mixture has to be the winner for me as it was just a lot, lot easier to paint with than the 1:2 ratio.