mist coat won’t stick to new plaster

[IfYouTolerateThis...]

The plaster is extremely smooth. That could be it, even though iv been over it once with sandpaper its still glass like. Should it be slightly rough before painting?

 

[EddieM]

The plaster is extremely smooth. That could be it, even though iv been over it once with sandpaper its still glass like. Should it be slightly rough before painting?

Yeah it needs a key..

 

[opps]

supermatt is a non-vinyl paint which is not durable. It is used on new plaster because it is porous and allows water vapour to pass through it to allow the new plaster to dry out. It is not suitable for wet conditions.

Agreed, it, and similar paints are often used inappropriately. I have seen other decorators use it in bathrooms on perfectly dried skimmed plasterboard. As soon as the steam builds up the ceiling goes from white to dark until the moisture evaporates.

Supermatt et al should not be used as a base coat. Period.

If you want to seal (dried) new plaster use a standard (diluted) matt emulsion. Only use Supermatt type paints if you know that the render/plaster is still wet. It/they are "bodge" paints used by people who need to paint something before it should be painted.

Sorry JohnD, I know that you know that but many other people don't.

 

[opps]

For the benefit of anyone reading this.

Do not follow the advice often offered up on the internet to thin emulsion by 50% water.

One thins paints so that the plaster doesn't suck the moisture out of the paint too quickly. Waterbased paints cure via a process called coalescence. In short, the water evaporates off and the remaining molecules shrink back and bond together. If you add too much water (read: 50/50) the coalescence process is compromised.

The whole point of thinning paint is to reduce the level of suction (ok- that is an oversimplification... nevertheless), but using a 50/50 mix does nothing with regards to suction. Yes, it can be useful if you want to see which areas need sanding prior to repainting, however if you do not want to sand significant areas back, just follow the thinning advice offered by the manufacturer (which is never 50/50). Thin the paint that much and you will end up with paint all over the place, it is extremely difficult to prevent it running down the walls, over the skirting, on to the carpet/etc.

Oh, and do not use Supermatt and the like unless you absolutely have to paint over wet(ish) plaster or (still drying) internal damp proofing cement render. Those paints are not suitable as base coats. You might get away with using them every now and then but if you use waterbased eggshell or vinyl silk over the top, you run the risk of opening a big fat tin of worms.

Sorry for the ranty post. I decorate for a living and unlike some supposed decorators, I want to provide a high quality finish. I could make more money by using those kinds of paints but I wouldn't be able to sleep at night.

 

[IfYouTolerateThis...]

Thanks for all the advice. Spoke to a painter today and he confirmed the fact that 50/50 is too thin and said that was the reason it wasn’t a strong bond. Advised covering what i’ve already done with two coats of full strength then the bathroom paint. Am going to give that a go with the paint i have on one wall and see how it is.
Defo going to hire a professional next time.