lime over gesso

[rebeccac]

We cleaned all the modern paint off our old walls and sanded them down to a local gesso flat plaster.

As an experiment we wetted this down very well then using a sponge worked over a thin coat of lime plaster -this was a Bio Calce intonacino which mixes NHL 3.5,marble and fine sand with a pozzolan.This was done over 2 years ago and is good with no craking or flaking.We haver now asked our local guy to do this all over the hallway but he is adament that he wants to use a type of adhesive first which would interfer with our walls breathability.He has done some plastering for use with lime before and his work was excellent.

As we have tried it out with complete success what would people think about telling him to go ahead and if it fails we are responsible?

I remember reading that if there is a chemical key between surfaces then lime plaster will adhere.

 

[Zebra]

I imagine he wants to use diluted PVA, as this will decrease the suction of the wall underneath, making it not only easier to apply the lime plaster over the top, but less likely that cracking will occur. If your substrate is very sucky, then applying an NHL plaster over it is tricky, as it goes off so fast. I had my plasterer in doing my bathroom while I was applying a NHL3.5 plaster to modern blockwork (very sucky) in the kitchen. I was wingeing and complaining about how quickly it was going off, and therefore how difficult it was to do, and he, despite being experienced in lime, said exactly the same thing to me: "I'd PVA that if I were you."
I didn't, and although it was difficult to do, it didn't crack. Of course, I had no reason not to. It was modern blockwork, but I'd done two walls already without, so wanted to carry on the same.
So, by all means tell him to go ahead without, but expect some wingeing. There's no reason however that the result won't be what you want. Keep it very wet, and spray it while it's drying.

 

[Penners]

As we have tried it out with complete success what would people think about telling him to go ahead and if it fails we are responsible?

I remember reading that if there is a chemical key between surfaces then lime plaster will adhere.

If, as seems to be the case, you are quite confident that the technique will work, then I can see no reason for you not to give the chap this assurance - in writing if necessary.

 

[Zebra]

If you use a plaster based on putty rather than NHL it will be much easier, although the results might be different.

 

[88v8]

Last time I saw a pro working with lime plaster, he had a pump-up sprayer and was wetting the ground [wall] as he went. It needs to be wetted until it won't suck any more, almost but not quite running.

Ivor

 

[Flyfisher]

Agreed.

I use one of these: http://www.screwfix.com/p/pressure-sprayer-7ltr/51100

 

[Cocoa]

First of all, let me emphasise that I am a rank amateur, with only one summer's worth of plastering experience!

Last summer, I did the same as you, although with far less prep. The walls of my ground floor were covered in about a hundred layers which I have diagnosed as limewash or casein paint - some was flaking off in big chunks, some adhered like a particularly tenacious limpet, so it made for a very bumpy wall. In the end, I skimmed over everything using a trowel, lime putty based skim, a lot of water, and very little science. A year later, I am still pleased with it!

I was trying to avoid doing the same thing on the hallway, because it's a lot of work and I'm not certain that the ladder I own is long enough, so I called Mike Wye to find if there was an alternative. The guy I spoke to was adamant that the technique I had used before wouldn't work and I would need a primer (although it has). He did suggest 'wakening up' the dead plaster by giving it about 20 (!) coats from a sprayer of very diluted lime wash, which was an approach that had worked for him but offended the lime purists.

Based on my (very limited!) experience, I would provide him with the assurance and go for it!

 

[Nigel Watts]

Are you sure your wall is gesso? Gesso is generally made with animal glue and a filler/pigment.