Lime Plaster Mix

[malcolm]

I'm nearly ready to start plastering in the kitchen onto a solid brick wall. Apart from a little repair in the attic this will be the first plastering I will have attempted inside. I have a tonne of plastering washed sharp sand (ordered in error) and am keen to use some of that up in my plastering mix. It doesn't have much fine stuff but seems to work in a 2:1 mix.

The kitchen needs to be insulated for building regs and I'm thinking straighten the wall with lime plaster, stick on wood fibre board insulation (or is it wood wool), more lime plaster on top for the final finish.

What do you use for lime plaster mixes indoors? Or could you direct me to more info - it must have been discussed at length on here but I can't find anything as plaster is too common a word and the search box refuses to search for it.

 

[Bevmarley]

Sorry cannot help but hubby is off to a one day lime plaster course 09th December so if I learn anything from him will let you know,

 

[ben_f]

I'm no expert but I do find that I keep doing bits of lime plaster indoors.

If you're adding insulation, why bother putting plaster onto the brick though? Always think about condensation consequences of added insulation. If the insulation material is impermeable (e.g. foam) you don't need to worry. If it is permeable, then you should ensure that the space between the indoor warm air and the bricks is ventilated from the outside, not the inside, otherwise you get water dripping down the inside of the walls, eventually causing problems.

The scratch coat (first coat) is the easy bit. I use a 3:1 of sharp sand to NHL3.5. Make it not too sloppy, wet the wall first and develop a throwing action of the plastering float to get it to hit the wall with a bit of inertia. It's good fun. Wear old shoes and some eye protection! Before the scratch coat goes hard, scratch it with lines no more than 3 inches apart as a key for the next coat. It doesn't need the key to stick to the base layer, but it does need it to enable you to push the next layer around to level it without pushing it right off.

Traditionally it would be 3 coats to the finish. This is where I always have difficulty and I've never attempted 3 coats, only 2 - maybe that's the problem, but I think that they get progressively more difficult as they are thinner coats. The thinner the coat, the more difficult it is to push the plaster around to achieve a flat finish. So I tend to get a lovely flat scratch coat, but finish a bit bumpy. For my finishing coat, I use fine sand (plastering sand or play sand) about 1:1 with the lime. If you need your hand holding, I'm in Bedford too and generally willing!

 

[malcolm]

I didn't realise there was anyone else from around here on the forum. I'm near the A1.

It's a good point - I ought to decide on the insulation. Building regs want the equivalent to 50mm insulated plasterboard (37mm celotex) which is roughly equivalent to 60mm wood fibre board. Instructions for that say put a layer of lime plaster on the brickwork to level it, glue the fibre board on with a permeable glue, then lime plaster over the top. The whole wall is solid but breathable so hopefully will show signs of damp it they get bad. I haven't quite figured out how the electric sockets will be mounted on the fibre board - whether it is stiff enough to mount the sockets in a recess.

I guess hair is not needed for any plaster coat hidden behind the plasterboard, so some sort of course sand mix with lime putty should be fine. Might risk some mesh on the fibre board joints then a finer mix.

 

[MatthewC]

I used Mike Wye's ready mixed 3:1 mortar for the scratch and float coats, followed by 2:1 plaster over that. Where I wanted a good finish there was a very thin skim of Regency plaster over that. My plasterer was pretty good but having done just one wall myself, I agree with you about the difficulty of the thin top coats. Also, you need strong arms...

Is the "plastering washed sharp sand" whch you have sharp sand or plastering sand? As I understand it, these are different: they have the roughly the same size particles but plastering sand has rounded corners whereas sharp has sharp edges which will make quite a difference when applying it (i.e. it won't flow so easily).

Can't help re insulation - we just plastered onto the 16" stone walls and it's lovely!

 

[Zebra]

I used 3:1 plastering sand: putty. I also levelled off the brickwork before applying woodfibre (the beige fluffy stuff). I applied it with oversize plastic washers though, and long screws. I know that makes a cold bridge, but oh well.
Internal wall insulation is obviously not as ideal as external, because of the dew point being located inside your wall. But if the structure is fully breathable I figure the moisture won't get trapped.
I too didn't bother with the third coat. I did the scratch coat, scratched it in a diamond pattern, then did a float coat (of the same mix), and sponged the float goat. I mean float coat. It is rather gritty looking, not very smooth. If you've bought sharp sand in error then maybe use it on the scratch coat?
I found it quite tricky plastering onto the woodfibre boards. The first time I wet them too much, and it just didn't adhere. Ended up taking it all off (it was only a small trial area) and doing it again.

 

[malcolm]

I'll order some lime putty (and some ready mix plaster just in case).

How did you mount electric sockets in the woodwool board Zebra? I half recall you might have mentioned screwing back to the brick. Any tips on channeling it for wiring would be useful too. I'm thinking a stanley knife.

 

[Zebra]

You can make putty extremely cheaply, by mixing a bag of hydrated lime in water in an old dustbin and letting it settle. Wants months to settle properly and get really creamy though.
I didn't chase out the wall. No need, and I didn't want to damage the brickwork. I chased out the back of the woodwool board with an old wood saw. The backboxes were screwed to the wall and the board carefully fitted over the cables. The box and the woodwool weren't affixed to one another, the woodwool was simply cut to fit closely around the box.
If your woodwool board is deep, consider using deeper backboxes than you normally would, otherwise you'll need massively long screws to screw the sockets on with.