New plastered walls - how many coats/preparation coats help

shd1970 · 22 Aug 2011

Hi everyone,

I am after a little advice so thanks in advance to anyone who has the time to reply.

I have had a new extension recently plastered.
New walls have been plaster boarded and skimmed, likewise with the ceiling.
The old original plastered walls have been skimmed too with some areas patched up prior to the skim layer.
Finally, some pipework has been sunk right in to the walls so they had what i would call a browning and skim layer.
The plastering was completed 2 weeks ago so what with the warm weather, i would say its quite a consistent pink colour.

Now then, my builder has said that the decorator will use a dulux paint but mix with "something" to make it porous to allow any remaining damp to come out.

Has anyone every heard of this ?
Will this make the walls any less durable ?
Will it still be able to be wiped ?

I'm not sure of the correct professional approach of prep layers that should be applied to a newly plastered room so if anyone can let me know the correct steps that i should expect as i'm paying for a professional job by my builder.

I am a little nervous as he has been desperate to finish the whole job so was concerned that he is trying to rush the paint job.

As i say, any replies are really appreciated.

Thanks

TheDec · 22 Aug 2011

There are paints such as contract emulsions that are applied in order to allow the walls to continue drying out, yet in your situation unadvisable. If the plaster skim is damp don't paint it.

Dec

misterhelpful · 22 Aug 2011

After two weeks I would imagine the skim is now dry but the chased areas may still be drying, depending on their depth.
You can use something like Dulux Supermatt or another contract matt as a mist coat, which is the first coat watered down by approx. 25%. These are non-vinyl paints which will still allow any moisture to dry out but are not really suitable as finish coats as they are not wipeable so you will still have to wait for any remaining moisture within the wall before applying your finishing emulsion (usually 2 coats).

I'm not sure what the builder is suggesting he will add to the paint but just make sure he doesn't use PVA as it can cause a whole host of problems.

shd1970 · 22 Aug 2011

thanks for the replies guys

so first thing then, is a good inspection with the decorator of the plastered walls and decide whether we both think its ready for painting

if so, then...

1 - first coat is a 25% diluted mistcoat of our chosen paint
2 - light sand
3 - 2nd coat of 15% diluted
4 - 3rd coat of 10%

....or something like that ?

i have noticed a few crack in the plaster.
at what point should the be filled / sanded please ?

misterhelpful · 22 Aug 2011

Fill imperfections after mist coating - they will show up better then.

There is no real need to water down subsequent coats by more than 5-10% and that is only to ease the application if required.

Richard C · 22 Aug 2011

Sean H said:
Now then, my builder has said that the decorator will use a dulux paint but mix with "something" to make it porous to allow any remaining damp to come out.

Never trust what a builder tells you; it’ll mostly be whatever bull sheite he needs to tell you to keep you happy

. After 14 days there won’t be a problem with the skim but the chases might still be a little “green” depending how deep they are; it may look dry but may still have a high residual moisture content.

I thin 25-30% tw-3 coats in quick succession; I say it helps adhesion, others say it makes not difference but there you go; let it dry overnight & then apply your chosen paint finish. I always thin emulsion at least 5%, sometimes more as it goes on easier & gives a better finish & I rarely use rollers. What’s with all this sanding? you don’t need to sand between coats & certainly not after the mist coats. If the finish isn’t good the paint is either too thick, your applying it incorrectly or using crap brushes; probably rollers too but, as I said, I rarely use them.

You should not have any cracks in a new skim 2 weeks after it’s been up; something has gone wrong, either with the preparation or the way the plaster has been laid on. Which is cracking the new boards or the re-skimmed original plaster?

TheDec · 22 Aug 2011

I fully agree with Richard, also if the Decorator is under the employ of the builder he will be well worded up with regard what to tell you, I think on your part an air of caution is needed.

Dec

shd1970 · 23 Aug 2011

thanks for all the input guys - really appreciated.

lets see what steps the decorator tells he is planning to do on thursday

I will report back once i know....

TheDec · 23 Aug 2011

Sean

As Richard has pointed out something has gone wrong with the plaster/plastering and you need to sort this out with the builder, If I were you I would forget about any redec until this problem has been resolved.

Dec

shd1970 · 23 Aug 2011

The crack in the new plaster is where the new extension wall (plasterboarded) joins the old wall.

The crack is about 12 inches on the old wall.
I remember that it wasnt just skimmed at this place.....it went down to the block work so the plaster will have needed to be deep.

I am not sure how deep nor whether they did it in phases (build up a rough browning layer first before adding te final skim)

If you imaging an upside down letter T, the lengths of the crack are 3" and 5"

Only done 2 weeks ago.........that aint right surely ?
I'm sure my builder will try to say it is.

Gotta say that i am getting nervous

Richard C · 23 Aug 2011

The cause of the crack is differential expansion/contraction where the two different wall materials meet. Unless it's done properly, you will never, ever stop cracks re-appearing where the two meet. There are ways of doing it but it requires more than just butting the two walls & reinforce taping the join. Also how thick is the base plaster & are you sure it's Browning, not many use it theses days.

You said cracks, is that the only one?

shd1970 · 23 Aug 2011

yes Richard only 1 crack....so far.........in the shape of an upturned T

I said browning only to indicate that because of the depth, i presumed that there may be a different product that is used for larger depth before a skim were applied.

Growler · 24 Aug 2011

misterhelpful said:
You can use something like Dulux Supermatt or another contract matt as a mist coat, which is the first coat watered down by approx. 25%. These are non-vinyl paints which will still allow any moisture to dry out but are not really suitable as finish coats as they are not wipeable so you will still have to wait for any remaining moisture within the wall before applying your finishing emulsion [u/](usually 2 coats).

That's a very important point Mr H makes and one which is often overlooked.

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New plastered walls - how many coats/preparation coats help

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