Ahh he's trade. He's passing down the knowledge from years ago. Maybe not needed now. I've plastered a few big holes in my time, as plaster just fell off the walls as I did jobs and have never EVER whitewashed them before painting. Just saying that's all.
Fair enough, probably OK on a small repair, but a whole wall i would personally do it. I believe its something to do with normal emulsion being too thick and drying before the fresh plaster gets a chance to absorb some thus creating a sealed base for your finished colour. Hence why its watered down.
Did a quick google: Below taken from here:
The mist coat is normally using the cheapest white or magnolia emulsion paint you can find, and then you water it down a bit. This miss coat then gets rolled onto the wall like you would be painting anything else. Make sure that you roll out any lines and what not as the dry plaster will pull the moisture out of the paint and will dry very quickly, and any lines left in the paint will need sanding.
The mist coat being diluted paint gets drawn into the wall and in essence, seals the wall. If you use undiluted paint, the moisture would get drawn out quickly, and the paint would sit on top of the plaster rather than become part of the plaster, if that makes sense. You may have seen in the past paint just pealing off walls in big sheets, and it looks like fresh plaster underneath and nine times out of ten this is because the plaster was painted without a mist coat.
I never recommend using any one coat paints as I find the extra coats of paint on a wall or ceiling the better it tends to look. All these one coat products never, in my opinion, get the high-quality finish I am after and in the end if you are spending all that money getting things plastered you may as well go that extra mile and finish them to a high standard.
