Paint on bare plaster

[Toughie]

Hi,

We recently had our bedroom painted and it seems our decorator used our bare plaster in the hallway to “roll out” the excess paint on his roller.

This was final coat paint and not watered down for a mist coat.

Do I need to scrape this off before a mist cost is applied? It has been done on multiple walls

Thanks

 

[Wayners]

No id leave it.
Not good practice to do that but I know decorators that don't worry about mistcoat.
On goes the paint as is.

 

[PerranOak]

Is the mist coat important?
Only we're having a new ceiling soon and we're going to do the decorating after the plastering's done.

 

[Wayners]

Trade paints often state 10% water max for first coat over plaster. Read online application data sheet.
Did they use trade paint?
Often with trade paints you need to add a splash of water especially in summer so do you know if that was done?
That paint may have been watered down a bit already.

This watering down 50/50 or using contract matt thinned as a mistcoat is from a bygone age and not recommended by paint manufacturers..
If that's contract matt is will sand off easy with a cheap orbital sander if that's what you want to do.
If that's a durable paint it's probably welded on firm so why bother doing anything.

That's why I said leave it. Not worth doing anything about.

Everyone should read the emulsion data sheet it's a basic A4 sheet and a easy read. All questions answered.
There are help lines as well.

 

[opps]

Is the mist coat important?
Only we're having a new ceiling soon and we're going to do the decorating after the plastering's done.

Water based paints cure through a process called coalescence. As the water evaporates off, the molecules bond down. Plaster is very porous. If you apply full fat emulsion over it, much of the water is sucked into the plaster rather than being able to evaporate off. The curing process may fail.

If the mist coat is over thinned or not thinned, the worst case scenario is that when you apply additional coats, the water in the new coat will soften the previous coats and the roller will pull the old paint off. The process of rolling creates a "vacuum", the air is expelled from the roller and it can pull the softened paint off.

As per @Wayners post, the first coat should be thinned based on the advice on the tin and not someone on YouTube. The (correct) thinning process is supposed to reduce the level of suction (read: porosity).

Another consideration is the inappropriate use of "contract" matt paints, eg Dulux Super Matt. They were designed so that they could be used over plaster that is still slightly wet. They have a very low acrylic/latex content. They allow the moisture in the plaster to escape. They do little with regards to the suction levels. Unfortunately, they are often used on dry plaster because the colour obliteration is superior. They are impossible to keep clean though and when using standard matt over them, you still have the suction problem.

I kinda understand your decorator using your wall to remove the old paint from his roller, but at a minimum, he should have misted the wall with water first.

Personally, if the colours were close, absent of washing the roller, I would have rolled it on a scrap of lining paper and then binned the paper.

I don't know your decorator, and, hopefully all will be code, but... in my opinion, that was sloppy.