Lime plaster (hemp actually) shrink/cracking at edges

16 May 2014
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United Kingdom
I'm using Ty Mawr, ready-mixed, fine-grade hemp plaster to do a base coat on an area of an interior brick and stone wall. Hopefully this picture links works ...View media item 97840
It's filling in area surrounded by good sound plaster (regular not lime) and going onto brick and stone. I applied it on a damp surface and I've sprayed it daily.

BUT after about a 1 week, it pulled away all around the edges and a crack of about 2mm has opened up when new meets old (as highlighted on the photo - but you can't really see it).

So what do I do? It's still a little bit flexible to the touch: do I add new plaster to the crack and push it in? Or leave it? Or try to push in the week-old plaster and hope it makes a new edge?

Lime plasterers - please help!!

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Why are you using expensive render instead of a simple sand and lime or sand and cement mix? Is it something that was at hand?
A little shrinkage and maybe cracking is OK on that base coat but any movement: "its still a little flexible" is bad news indicating a wrong mix or a bad prep.
Best practice would be to square it off & take it all off back to masonry.
And then prep using SBR and a scratch coat.
Cut your edges to plumb & level lines so that you are working to clean edges.
Lightly scratch and wait 24hrs before applying the next coat.
Stay a few mm's below the finished surface to allow room for your skim.

While doing this work, why not remove the surface outlet & sink a DSS into the side cheek of the chimney breast?
Is that vent an air brick or a shuttered vent?
Did you sweep the flue before blocking it up?
Sorry mate but if you're asking why I'm not using a sand and cement mix nor understand why a lime hemp base cost isn't still a little flexible after only a week, then you're not the lime expert I need. Thanks for helping anyway.
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Just came across this post. Hemp plaster is probably a bit more tricky to work with compared to Natural Hydraulic plaster which dries and solidifies quicker. I would say lime putty plaster should not be sprayed with too much water or it will be more prone to cracking as it dries. Having said that, it's normal that cracks appear where new plaster of a different texture meets old. Your plaster should be reasonably hard by now. I would give it a gentle knock with the back of your fingers to make sure the whole plaster has glued to the wall (in case an area sounded hollow that would mean the plaster has not adhered properly to the brickwork/stone). A few cracks due to shrinkage is normal, but you could rake gently with the end of a cold chisel the sections where the cracks are 2mm thick to make them slightly wider and fill the cracks with new plaster. Ty-Mawr can also give you some info.

If you haven't applied the final coat yet and only if you want it smooth, apply pressure with the trowel once the plaster start to harden.

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