New lime plaster is brittle

[GillesF]

Hi all

Last Saturday I tried lime plastering for the first time, based on this video. I used a lime plaster mix using 1 part lime putty (DIY from hydrated lime) and 3 parts sharp + horse hair. I then sprayed water about 8 times on the area I was going to apply a scratch coat to. The mix was quite dry so I had to put some effort on applying it, but otherwise the work went pretty well.

We're now 4 days later and the lime plaster has become quite hard. However, I have noticed on the bottom of the scratch coat (near the floor) that I can simply pull the lime plaster with my fingers from the wall. I can also easily crumble the dried lime plaster between my fingers. I assume this is not normal? What could be the cause of this? I was thinking that the wall might not have been damp enough so it pulled all the moisture out of the plaster mix but that seems weird after spraying about 8 times that morning before starting plastering? Is it also possible that it dried too fast? There was a lot of draught coming from the cellar between the wall and the floor boards and blowing right on top of the plaster.

How should I proceed now? Remove all the new plaster and start all over? Or carefully check which parts need to be replace (how best to check?)?

Thanks for the help!

 

[Cubist]

Hey Gilles,
It could be a little too soon to worry. 'Picking' at exposed sections of new plaster, even weeks after it's been applied, will result in some of it coming away from the surface. Provided it's not cracking and pealing away in sections it's best to leave it alone for a few more days until you're ready for any additional coats.

Plasters, lime and gypsum based, behave a little counter-intuitively. Wetting the base surface before applying the first render coat operates to limit the base materials ability to absorb water from the plaster. Too fast and the plaster fails to secure a good initial bond to whatever material comprises the wall and consequently it can fall away in sheets. If either the wall or the plaster is too wet, it may sag and easily ripple while you're floating it out. Too dry and it won't stick at all.

It's a fine balance, and it could be that your mix was a fraction too dry. It may be an idea to mist the surface a little. As suggested above, leave it alone and give it a little time. The worst that can happen now is that when you try to float the next coat the render coat below may pull away.

Hopefully other members with more direct experience than I will soon be along to either confirm the above or scathingly dismiss it along with a better suggestion concerning how to proceed.

Cheers and Welkom,
Cubist

 

[aockenden]

Are you sure that your non-hydraulic dry hydrate lime powder was fairly fresh when you turned it into putty? If it was some old stale crap that had been slowly carbonating in the bag via air moisture for a long time then it’ll be weak and crumbly / chalky.

 

[GillesF]

Are you sure that your non-hydraulic dry hydrate lime powder was fairly fresh when you turned it into putty? If it was some old stale crap that had been slowly carbonating in the bag via air moisture for a long time then it’ll be weak and crumbly / chalky.

I checked this when I purchased them, they were only a few months old and all bags were undamaged. I made the putty a few days after purchase.

 

[GillesF]

Hey Gilles,
It could be a little too soon to worry. 'Picking' at exposed sections of new plaster, even weeks after it's been applied, will result in some of it coming away from the surface. Provided it's not cracking and pealing away in sections it's best to leave it alone for a few more days until you're ready for any additional coats.

Plasters, lime and gypsum based, behave a little counter-intuitively. Wetting the base surface before applying the first render coat operates to limit the base materials ability to absorb water from the plaster. Too fast and the plaster fails to secure a good initial bond to whatever material comprises the wall and consequently it can fall away in sheets. If either the wall or the plaster is too wet, it may sag and easily ripple while you're floating it out. Too dry and it won't stick at all.

It's a fine balance, and it could be that your mix was a fraction too dry. It may be an idea to mist the surface a little. As suggested above, leave it alone and give it a little time. The worst that can happen now is that when you try to float the next coat the render coat below may pull away.

Hopefully other members with more direct experience than I will soon be along to either confirm the above or scathingly dismiss it along with a better suggestion concerning how to proceed.

Cheers and Welkom,
Cubist

Hi

Thanks for all the info! I'll be a bit more patient then

Just out of curiosity: what could I do to lessen the suction from the bricks then? Is it possible to use the water on top of the putty (not sure how it is called) and use that as a sort of primer? Like a very watery limewash?

 

[a twig]

Because it’s central heating season which can cause the plaster to dry out too quickly, in the past I’d mist the plaster with a hand sprayer over the following few days, but then I was being lazy and doing two thicker coats rather than the three coat method.

Having ploughed through that video he’s pretty spot on so if there are issues you’ve potentially answered your own question with the mix being a bit dry.

If things do start to crack and crumble applying limewash will help repair it, due to lack of faith in my own plastering ability even on walls where I was planning on clay painting I’d usually do a coat or two of limewash first

 

[GillesF]

There's no central heating in this room at the moment, temperature is about 18° I think. Unfortunately I cannot install heating as long as plastering is not done yet
I was unsure about the misting after plastering, I though this was only done when pointing. Will do that next time. Do you believe hessian is helpful too?

Yes, the video is great, learned a lot from it! I did notice his mix is a LOT "smoother" than mine.

 

[a twig]

I’ve never used hessian for internal work, always do for anything external though.

He was using pre-mixed lime plasters from Ty Mawr, makes the job a lot easier if you have space in the budget, I often did the same when in a rush, wife lost interest in labouring for me and wheelbarrow / bucket mixing gets old quickly when you’ve a lot to do. Chuck the premix bag into a bucket, give it a mix and away you go.

3 sharp to one putty is a pretty coarse mix for internal work as well. I usually did 1 sharp : 2 builders : 1 putty for base coat then 3 plastering sand : 1 putty for top coat

 

[aockenden]

I checked this when I purchased them, they were only a few months old and all bags were undamaged. I made the putty a few days after purchase.

K that’s probably not the issue then. Non-hydraulic limes are softer than NHL’s, so being able to crush a chunk between your fingers isn’t the end of the world. A very hard compression between two fingers will crush most of the mortars and plasters I make.

 

[GillesF]

3 sharp to one putty is a pretty coarse mix for internal work as well. I usually did 1 sharp : 2 builders : 1 putty for base coat then 3 plastering sand : 1 putty for top coat

Don't you mean 1 plastering and : 1 putty for top coat?

Thanks for the info all. My next plan is to:

1) Wait long enough to have it cure and dry completely as mentioned by Cubist
2) Check this part of the plaster and see if it's still brittle

If still brittle:

1) Remove this plaster
2) Wet the wall even more than last time
3) Use a slightly smoother mix by adding a tiny bit of water
4) Block air passing through the floor from the cellar to prevent rapid drying
4) Keep the lime plaster moist after application by misting afterwards

 

[a twig]

Don't you mean 1 plastering and : 1 putty for top coat?

No, I used to stick to 1 putty : 3 plastering

 

[paulc]

Going to a 1:1 mix of sand & putty, it has to go on very thin or the plaster will crack. The sort of thickness will be 0.5mm or less. For a thicker finishing coat, use a mix between 2:1 and 3:1.

 

[GillesF]

Thanks for the tips and clarification. I tried skimming with a 1:1 mix and it would indeed crack a lot.