Decorating Advice

[9fingers]

Our house is 92 years old and internal walls are 3" cinder block with lime mortar render and skim coat plaster.

The wall in question is part of our walking pantry and will have been dry and cool (north facing). One wall has had a few coats of white emulsion that has very poor adhesion but no has come the time to spruce things up in there and I've scraped the emulsion off back to bare plaster which is superb condition. I cannot see why it would ever have been re-plastered so it is probably 92 year old plaster.

I've seen MikeG refer to mist coat of emulsion applied to new plaster but is this also the correct way to prepare these mature plaster walls. I want to get this right first time as the wall with have a matrix of spur bracket shelving screwed onto it which considering the friability of the old cinder blocks and lime render will only go up once and be a PITA to repaint after installation.
I've read some suggestions of dilute pva but that will form a skin and the emulsion will slide all over the place on top so I don't rate that as a solution.

So mist coat it or something else?

TIA

Bob

 

[Phil]

Bob, if you have done a good job of cleaning back to a 'good 'n healthy' base, then I would use a Universal undercoat applied to ensure 100% coverage, let it cure for 24 hours and then paint the first coat of whatever you want to put on. I prefer to wait 24 hours between coats.
Normal PVA breaths but a Plascon Double Velvet or similar seals and can lift more easily.

My 2 cents FWIW

 

[RogerS]

Bob, if you've gone back to bare plaster then, yes, I'd say definitely a mist coat.

 

[9fingers]

Bob, if you have done a good job of cleaning back to a 'good 'n healthy' base, then I would use a Universal undercoat applied to ensure 100% coverage, let it cure for 24 hours and then paint the first coat of whatever you want to put on. I prefer to wait 24 hours between coats.
Normal PVA breaths but a Plascon Double Velvet or similar seals and can lift more easily.

My 2 cents FWIW

Thanks Phil,

Looking online there seems a world of difference between ZA universal undercoat which is an oil based paint specified for lots of different surfaces including plaster and UK universal primer intended for both wood and metal and is water based ( hawk - spit - ding!!)

Bob

 

[9fingers]

Bob, if you've gone back to bare plaster then, yes, I'd say definitely a mist coat.

Yes it is bare plaster, stable and very dry. I has no trouble scraping the current emulsion off - it was falling off in places.
I think normal mist cost to put on fresh plaster is 25% water, I wonder as this is so much drier and possibly porous if I should use even more water to thin the first coat?


Bob

 

[Rod]

90 odd yrs ago, wouldn't they have used distemper?

Rod

 

[Mike G]

Distemper or a limewash, neither of which go well with modern paints. However, if there isn't any remaining, and it really is bare plaster, then yes, apply a mist coat. Make this as wet as you practically can. If it's too wet it will run down the roller/ brush and up your arm. You want it just slightly less wet than that.

 

[Mike G]

......Yes it is bare plaster, stable and very dry. I has no trouble scraping the current emulsion off - it was falling off in places.......

There's a clue here. This sounds like the emulsion was painted over a non-compatible "paint", either limewash or a casein-based paint. If you wipe the surface of the wall with a damp sponge, does anything come off the wall onto the sponge?

 

[9fingers]

90 odd yrs ago, wouldn't they have used distemper?

Rod

Quite possibly but the paint thickness of the flakes I've taken off is about 110 microns thick.
Even allowing for us being in the house since 1982 and never having painted this wall (I'm not a fan of decorating!), I think a previous owner must have stripped back the 1920s finishes and used modern(ish) matt emulsion.

As a child I seem to remember my dad trying to paint over distemper and failing and having to strip off the distemper before starting again but that would have been nearly 60 years ago so i can't be sure.

Bob

 

[9fingers]

......Yes it is bare plaster, stable and very dry. I has no trouble scraping the current emulsion off - it was falling off in places.......

There's a clue here. This sounds like the emulsion was painted over a non-compatible "paint", either limewash or a casein-based paint. If you wipe the surface of the wall with a damp sponge, does anything come off the wall onto the sponge?

There are a few odd areas where the plaster is not (yet) completely bare. If I sponge those areas, the sponge goes white and the plaster is cleaner.

So bearing in mind how thin the paint flakes are (110um)**, I think the wall has been scraped "nearly clean" by a previous decorator but not washed down or mist coated when the modern matt emulsion was put on. Hence the ease of scraping it off.

I think tomorrow will be a messy wash down until the sponge stays clean, allow to dry, then mist coat.
Is a mist coat of silk emulsion OK? as that is what I'll be finishing with.

Bob

**That would be only just over 1um per year!!

 

[Mike G]

......I think tomorrow will be a messy wash down until the sponge stays clean, allow to dry, then mist coat.
Is a mist coat of silk emulsion OK? as that is what I'll be finishing with.........

Yep, that's the way. Silk emulsion is fine as a mist coat. In fact, if the walls are wet from the sponge-ing down, then you won't need to water the paint down so much.

 

[9fingers]

......I think tomorrow will be a messy wash down until the sponge stays clean, allow to dry, then mist coat.
Is a mist coat of silk emulsion OK? as that is what I'll be finishing with.........

Yep, that's the way. Silk emulsion is fine as a mist coat. In fact, if the walls are wet from the sponge-ing down, then you won't need to water the paint down so much.

Thanks Mike

Paint on order from toolsatan as I write.

Bob

 

[Rod]

I redecorated a flat we rented back in 1967 with emulsion - I hadn't realised that it had previously been limewashed!
Fortunately for me, other tenants in the building had complained to the Rent Tribunal and the flats were due for an inspection - the Landlord in a panic brought in decorators and put it right at his expense.

Rod

 

[Phil]

Looking online there seems a world of difference between ZA universal undercoat which is an oil based paint specified for lots of different surfaces including plaster and UK universal primer intended for both wood and metal and is water based ( hawk - spit - ding!!)

Bob

Bob, I have only used the oil based one over the years. Drying time not too bad - couple of hours and it also takes tinting.

There is another one which is spirit based for use on new raw plaster. I used at son & DIL house where they had new windows installed with plaster around on the outside of the house. Works well and being spirit based dries quickly.

I agree with the washing down till the water is clear. Our first house was plaster and paint outside and I had to use a wire brush to get off all the loose stuff (by hand, now I would use the angle grinder with the steel brush)
The outbuildings were painted with a distemeper mix or a whitewash. That took a lot more effort, sweat and langauge to clean up for painting.