Lime Skim / Finishing Coat Questions

[aockenden]

Well after some long-lasting sidetrack projects in our extension we are finally back to some more plastering in the old house. We’re getting our second insulating lime, float-coat of plaster onto the walls so I need to start thinking about our finishing / skim coat as that’ll be just around the corner.

We’ll be using our own homemade lime putty which is ready to go, but I’m struggling to find a fine plastering sand which looks good. Where is everyone getting theirs these days? Ideally I’d like something washed, as I’ve been burned by cheap salty aggregate in the past and the last thing I want is efflorescence on our newly skimmed walls.

Also what ratios of putty to sand have been the most noob-friendly for everyone? I was going to start with 1:1 putty to sand but not sure really.

We’ll be finishing with a fine-grain sponge float to start with and if that’s not smooth enough we’ll follow-up with a stiff but slightly flexible plastic trowel / float thing.

 

[malcolm]

Plastering sand is going to be a nobody knows question. I used to buy a local name sand from the builders merchant which was a sharp sand with a fabulous mix of fines and course. When I went to get some more they had changed supplier and it was a washed sharp sand with a consistent grain size. Guess modern plasterers wanted something different.

For a skim coat the local builders merchant sold me fine silver sand which worked well for a skim coat. But the version I got was a very fine grain sand and the internet suggests the silver sand name is used for all sorts of different grain sizes and types of sand.

Probably best bet is to go into the builders merchant and see what they suggest and have a look at it. Otherwise the sand or pre-mix you can buy from the lime suppliers is probably more reliable if heavy on delivery costs.

 

[aockenden]

Ok well that sounds like it’s worth doing to stop by the local builders merchants. I’ve never heard of silver sand before.

Any thoughts on ratios of putty to sand?

 

[paulc]

Two parts sand to one part lime putty.
I've used fine ground marble & putty before now for a fine finishing coat. Ground marble is expensive though - Chalk would be a cheaper alternative.
What ever you use, it is worth sifting before mixing. A flour sieve from the kitchen is good when you need relatively small quantities (up to 25Kg).

 

[aockenden]

Two parts sand to one part lime putty.
I've used fine ground marble & putty before now for a fine finishing coat. Ground marble is expensive though - Chalk would be a cheaper alternative.
What ever you use, it is worth sifting before mixing. A flour sieve from the kitchen is good when you need relatively small quantities (up to 25Kg).

Ok cool and where did you get ground marble / chalk? It doesn’t sound like something you’d find at the local bodgers-merchant?

 

[paulc]

Ok cool and where did you get ground marble / chalk? It doesn’t sound like something you’d find at the local bodgers-merchant?

Tiranti Marble Filler Powder

100mesh white marble filler powder (technically calcium carbonate - CaCo2). 150 microns is the largest particle size, but 50% gets through 100 mesh

www.tiranti.co.uk

https://cornishlime.co.uk/products/associated-products/marble-dust/ - Also does chalk.
https://www.cornelissen.com/pigments-gums-and-resins/chalk-fillers-and-abrasives.html - Whiting is the fine chalk powder.
https://www.needhamchalks.co.uk/ - If you need larger quantities of chalk, although they will do 25Kg bags if asked.
Some garden centres may stock ground chalk for liming purposes, as might a local agricultural supplier.

 

[aockenden]

Tiranti Marble Filler Powder

100mesh white marble filler powder (technically calcium carbonate - CaCo2). 150 microns is the largest particle size, but 50% gets through 100 mesh

www.tiranti.co.uk

https://cornishlime.co.uk/products/associated-products/marble-dust/ - Also does chalk.
https://www.cornelissen.com/pigments-gums-and-resins/chalk-fillers-and-abrasives.html - Whiting is the fine chalk powder.
https://www.needhamchalks.co.uk/ - If you need larger quantities of chalk, although they will do 25Kg bags if asked.
Some garden centres may stock ground chalk for liming purposes, as might a local agricultural supplier.

Thanks much I’ll take a look!