www.ultimatehandyman.co.uk

I had some walls plastered well over a year ago and on the advice of a local builder have applied two coats of Leyland Trade, Super Leytex, High Opacity Matt white directly to the plaster by roller as an undercoat.
The paint states "Ideal for work on new plaster, the permeable film has the added benefit of allowing new plaster surfaces to breathe and dry out", but the finsih looks very patchy and applying more paint by roller or brush does not seem to help.
The pictures below hopefully show what I mean, and would really appreciate any advice how I can correct this before the two top coats of another colour are applied - trying to avoid a terrible final finish.

Many thanks
Keith

I suspect the problem is that you did not do a diluted first coat. New plaster will suck all the water from the paint so it does not stick to the surface properly. Hopefully Dynamod on here will pick this up and advise on a remedial process. This might involve a sealer like Gardz to allow other coats over but wait for him to advise.

DWD

dewaltdisney wrote: ↑Thu Aug 29, 2024 11:21 am I suspect the problem is that you did not do a diluted first coat. New plaster will suck all the water from the paint so it does not stick to the surface properly. Hopefully Dynamod on here will pick this up and advise on a remedial process. This might involve a sealer like Gardz to allow other coats over but wait for him to advise.

DWD

I asked the builder about a mist coat and was advised not to...
Thanks for your response, hopefully if Dynamod sees the post he can help further
Keith

Morning folks

OBSERVATIONS.

Although certain manufacturers do say their contract emulsion can be applied undiluted to new plaster, I'm not entirely convinced this is a wise or even correct practice. Mist coats (a thinned out contract emulsion coat) traditionally are thinned to around 20% water dilution by volume, for the reason already stated. That figure can vary slightly either way, but not by much.

You seem to have a strong directional light in the room, which will pick up defects in the surface more readily. Think inspection lamp along car bodywork to highlight dents here. New plaster is seldom a uniform texture/porosity, so a strong light will pick this up.

A smaller roller head (say 9") will not cover the area as rapidly as a 12" or 15" head, and the finish may dry out a bit rapidly during application, leading to a patchy look.

Contract matt, despite being an emulsion, isn't really considered a finish paint, and vinyl matt is a much better choice for most situations.

Sealer coats, such as Gardz or Peelstop (by both Zinsser) are only really required if there's visible evidence of the surface coating showing signs of crazing or flaking, indicating the water content of the first coat got sucked back too rapidly and has left a layer of (partially) unbound solids. Have a close look at your surface for this, as it's not apparent from the photos, but a coat of Peelstop would be recommended if there is any of this effect showing. Another coat over an unsealed, crazing base coat could easily come off with the pressure of the roller. If it's intact, there shouldn't be an issue beyond the cosmetic appearance of the surface.

WHAT TO DO

In this particular case, I'd do the following.

Get a decent TRADE vinyl matt such as Johnstones or Crown. The wall will be getting 2 coats of this paint.
Thin the first coat slightly, say 5% or 10% over the contract matt. Contract matt is absorbent, so this will prevent it drying in too rapidly and promote adhesion. Don't over-thin this coat.
Apply a full strength coat of vinyl matt once the preceding coat is fully dried.
Keep heating off and shut the windows (it increases humidity and prevents the finish drying too rapidly)
Use a larger roller as previously stated to get the paint on faster.

Sorry if this comes off as a bit anal, but it not 'just painting' as a lot of people seem to think

Wow, thank you Dynamod for the full, in-depth observations and your recomendations on how to try and fix it - which I will definately be following.
Are the 2 coats of the trade vinyl matt the top coat colour - or do I need to apply some more white first? - sorry if that seems a daft question.

Now realise its not 'just painting' and should seek better advice than from the builder in the future

Really appreciate your help

Sorry for the late reply, but been out of signal this weekend.

The wall is ready for vinyl matt (assuming no cracking or crazing of what is on) Apply as per above

No worries, thanks for the reply.

It will be a week or two before I can get back to this and will let you know how I get on.

Thanks again