Gypsum basecoat, lime topcoat?

[Zebra]

My front extension is in modern lightweight blockwork but I want to do a lime plaster anyway to blend it in better with the old house. A "fat" lime plaster straight onto blockwork isn't a good idea because the joints show through. I know that an NHL base coat and fat lime topcoat would do the job, but I can't readily get hold of any NHL. Has anybody tried a gypsum ("hardwall") base coat under a fat lime top coat? Breathability isn't an issue.

 

[88v8]

Presumably you'll scratch the basecoat?
Provided it's is suitably wetted when you apply the lime, don't see why it wouldn't work.

Ivor

 

[skier-hughes]

Why not a haired base coat and then your top coat?

 

[Zebra]

Well, perhaps it's wrong, but I was told, when I was doing the external render, that if you use fat lime over modern blockwork, the joints in the blockwork will show through. I've done an experimental patch indoors, and they kind of do, not a lot, but they do.

 

[plasticpigeon]

Why do you need a lime top coat? if it is going to be left unpainted perhaps white cement and sand would also work, or if to be painted why a lime skim, wouldn't a limewash or other natural paint finish stick to a modern plaster? I think lime is a great product, and use it for building as the bricks on my house are rather soft, but it is certainly more fussy to use thazn modern products.

 

[Zebra]

I just wanted to match the look of the plaster on the rest of the house. Yes it will be limewashed. Besides, the ceiling is done now, in lime.

 

[88v8]

I suspect the issue with plastering on blocks is the difference in permeability between the blocks and the cement mortar.
Says here to build the wall with NHL mortar.
http://www.stastier.co.uk/nhl/guides/renblks.htm

Provided your undercoat is uniform you should not get show-through.

Ivor

 

[plasticpigeon]

if it is any help at all, in my house the ceilings and basecoat plaster on the lintels I believe is a gauged lime plaster with gypsum in. Then lime plaster was put over the stronger gauged plaster on the lintels and it seemed to stick as well to that as it did to the bricks. It is just that the gauged plaster is a lot stronger.

 

[Zebra]

Aha! Yes, I'm familiar with that St Astier guide Ivor, which is where I got the idea that the ideal way to do it would be an NHL render. I could order some in and pay the delivery charge I suppose.
The wall has a cement mortar, there's nothing I can do about that, and yes, I think the difference in porosity between the extremely porous blocks and the cement mortar is why it's showing through.

A guaged lime plaster - I'd not heard that term. Google suggests it simply means a lime mix with cement added. You mention though, plasticpigeon, a gauged lime plaster with gypsum in. This sounds like the very thing! Any idea of ratios? I'll do some hinternet research!

 

[Zebra]

Ooh, I've found some information here.
http://www.buildingconservation.com/articles/limegauging/limegauging.htm
It suggests that any gauging will lead to a mix which is unpredictable, so I'd be taking my chances!

I've also found some info here: www.womersleys.co.uk/pdfs/limeplastering.pdf

"...it is suggested that you use a similarly haired lime plaster gauged with 10% gypsum, casting plaster, by the volume of the binder. It is likely that it was originally gauged with a large proportion of gypsum but the lower grade gypsum used
then is no longer available in this country."

 

[Zebra]

Also found this on buildingconservation.com:
"...Internal putty plasters have been and still are on occasion gauged with gypsum to accelerate the set and reduce shrinkage. This was commonly done from c 1760 to ceilings and especially run and cast work, but never to walls where there is a risk of damp as gypsum is slightly soluble in water and sulphate salts migrate and crystallise on the plaster surface...."

However, I think I might follow Ivor's advice after all, and simply use a putty plaster for both coats. On my test patch, the joints don't seem to show through once I've got the top coat on, and gauging seems a little unpredictable. I don't think it needs hair though, does it, on a wall which is structurally stable?

 

[Flyfisher]

Our block work kitchen extension was covered in a basic NHL render with no hair. I then lime washed it to match the rest of the house. Texture is slightly different to the other parts of the house, but this is mainly because of the different construction. I don't think I'd want a seamless match anyway. The block work doesn't show through the render, even after the heaviest rainfall.

 

[plasticpigeon]

It's all a bit baffling! I lime plastered my kitchen with NHL/hydrate mix with plastering sand for the render, and a ready mixed putty lime top/skim/setting coat from Ty Mawr. I troweled it on rubbed with a sponge float and then went over again with a steel trowel to get it really smooth. A couple of coats of clay paint and i would defy anyone to tell it wasn't plasterboard!!!! What finish differences do others find between different plasters?

 

[Zebra]

Elsewhere I've just done a plastering sand/putty mix for the top coat, and sponged it, without trowelling again. The sponging leaves it gritty. Then I limewash that. It looks much more rough and "organic" than plasterboard.

 

[livias]

Gauged lime plaster for interior work is 6 parts lime putty mortar to one part pure gypsum aka plaster of Paris.

Pure gypsum goes bad with damp or age just as do other gypsum plasters and very few people use it so it is hard to get hold of. You can try Tiranti, or (Southwestern) Industrial Plaster both of whom will do bags by mail order. I get mine from a specialist plaster supplier in Norwich. Any company which produces gypsum cornice will use pure gypsum so might sell you some or point you in the direction of a local supplier either of which should be cheaper than mail order

Gauging lime plaster with gypsum is a great trick of the trade for indoor work as it minimises the amount of tending which is needed. The gypsum sets fairly quickly (far too quickly in fact, so add a retardant), providing a matrix to hold the lime plaster in place as it sets more slowly. Keep the gypsum in a sealed box in a dry place and use it within a couple of months

As for NHL. I am surprised that you cannot find any, Zebra. Try ringing around the local builders merchants and one of them is likely to stock NHL 3.5. Again, it is not greatly in demand and does go off, but it does get used by builders and so is much more widely available than is pure gypsum

 

[Zebra]

Thank you livias. That's very useful. I shall not bother to try to get hold of any pure gypsum. This is after all a newly built part of the house.
I had a very rapid but intense conversation with the Old House Store today. They seemed very keen to get me off the phone, there must have been lions outside. Anyway, the gist of it went: Can you use fat lime topcoat with NHL basecoat? Noooo. Can you use fat lime over modern blockwork? Nooooo. Should I use NHL3.5 then? Yes. Buy it online.
Decision made then. Stuff ordered. Thanks all.

 

[bennoa1]

I get my NHL3.5 from Travis Perkins who have 600 branches nationwide. You have branches in St Albans and Brickett Wood but maybe there is one closer so check their web site. You'll need to check availability beforehand as they don't tend to hold much over the winter months, if any.
bennoa1.

 

[MatthewC]

I get my NHL3.5 from Travis Perkins who have 600 branches nationwide. You have branches in St Albans and Brickett Wood but maybe there is one closer so check their web site. You'll need to check availability beforehand as they don't tend to hold much over the winter months, if any.
bennoa1.

That's helpful but annoying. I am fairly well known in my local TP but when I asked about NHL they said nooooo (just like Zebra!), so I have had to go (several times) to the not so convenient and possibly expensive Old House Store who have a branch in Woodstock (only 30 miles return). I'll try TP again next time.

 

[Zebra]

Ironically, since Old House Store gave the advice, they were out of stock, so I ordered it from Ty Mawr in the end. I have tried to get it from Travis Perkins in the past and they sort of said "Eh?" (I'm paraphrasing...) but I admit I didn't try them this time. Oh well...

 

[livias]

Just checked and yes its Travis Perkins where I get my hydraulic lime (NHL3.5) too. The yard staff in my local branch are fairly clueless about product and more than once I have had to stop them loading hydrated lime into the van instead of hydraulic lime