Does anyone know how to make a lime/cement mortar that could be used on an internal wall like a sort of very thick paint?

[Beachcomber]

OK. Will try to explain this clearly!

We are renovating an old French house and there is a room off the kitchen which has once been made of stone walls with a stone vaulted ceiling.

The previous owners made a sort of cavity wall of siporex on top of the stone (ahhh sacrilege). We have demolished a large part of it and we have done a traditional lime/sand mortar on some of the walls.

We are, however, left with one wall that it would be difficult to demolish (pluming, electrics etc hidden in the cavity). This (siporex) wall has been painted with an eggshell synthetic kitchen paint.

I would like to do a very light mortar/render (sorry don't really have the vocab in English) on this wall. Basically I want it to look like a lime rendered wall but I can't lime render it because the render won't stick on the paint.

I would like to make something similar to this sort of product.

If I mix lime with white cement and possibly some fine grain sand will I have something that will do the job?

I want it to be something that can almost be painted on the wall but that will be thick enough to cover imperfections.

I know I could just buy the product in the link but we have a big surface to do and it will be expensive (also we like to make things ourselves when we can).

Many thanks for any help....

Forgot to say - I don't want the wall to be white but a very very light sand or 'string' colour.

I imagine this could either be achieved by the addition of fine grain sand plus perhaps a powdered pigment.

Any thoughts?

You say the render won't stick to the paint.

Over here we would probably scutch the surface (chip it) then apply a coat of builders' PVA, let it dry, then a coat of the PVA mixed with cement, then render onto it. In some cases fix expanded metal lathing to the wall as well to hold the weight of the render. You would look for instructions on the website of the PVA company (e.g. Unibond) or a maker of Lime Plasters. Ordinary PVA is not waterproof so may fall of if it becomes damp, but you can get a waterproof variant or an alternative called SBR.

I think you are describing either a lime-wash or possibly a distemper. Lime wash is inclined to flake and dust off, distemper is very distressing to try to redecorate on, and smells of boiled-up horses' hooves.

You probably need to consult someone who knows about restoring old buildings, there may be a Conservation officer at the Hotel de Ville or an association for the preservation and restoration of old buildings.

People who work on old houses using lime don't approve of cement. Lime needs to be able to react with the carbon dioxide in the air to harden, and to "breathe" so damp is not sealed in with impervious paint.

It is a speciality that I haven't learned.

Thanks for your answer PigletJohn.

Basically we would like not to have to chip the current surface if we can avoid it - we could just paint on top of it but it would look better and go better with the rest of the room if we do something that has a bit of depth to it.

We are doing up this room to sell the house and we have a lot of other things to do so don't want to spend too much time on this one wall.

I agree that cement is a bad idea in general in old houses. However this wall has already been sort of ruined by the previous owners - it doesn't need to breathe as such as it has already been covered over with ventilation in place. Either we just add synthetic paint to the covering that is already synthetic or we try to add on a covering that has a natural finish.

Perhaps I want to have my cake and eat it

If we had more time we would demolish the syporex/painted wall and do a proper render. But that would mean encasing (probably not the correct term) the plumbing and electrics, and we just don't have the time to do that.

I will look into your suggestions and try to get advice from a builder friend who renovates old houses.

The room is slightly damp so we don't want to do something just for the sale that will fall off the wall/crack in a few months time as I feel that would be very dishonest with regards to a future buyer.

Just to be clear - the product I am hoping to use would not be applied onto the stone wall (if we were on clear stone we would lime render it). It is on a cavity syporex emulsioned wall (with vents already on place in order to ventilate the stone wall).

It is hard to describe, especially as my DIY vocab is mostly French!

Is syporex a lightweight concrete blockwork wall?

You could still try spab, or limetech or st astier technical helplines for surface prep advice for rendering.

Also having re-read your post - is the paint directly on to the wall so you can see the block lines? Is this why you want "something with depth to it"?

In which case can't you use lime render to match the rest of the room but on expanded metal lathe screwed to the wall
Like piglet john mentioned? That will provide a mechanical key on the wall without the need for chipping or paint removal?

Is this the sort of thing?

'tis expensive but my DH swears by it - in fact, I think it is holding up most of the walls in our (timber framed) house.

www.lime.org.uk/products/wall-paints/internal-surface-preparation/beeckotex

Yes, syporex is a sort of light concrete block - I think it is what people used to insulate a wall before plaster board and modern insulation existed.

No you can't see the block lines - the syporex has been plastered and then painted. They have actually done a very professional job of it, but it is very very ugly and not at all fitting with the room.

Rockdoctor, that is close to the effect I want to create - I just want to mix the paint/render myself.

Have been looking at lime/milk/plaster mixtures and going to try something like that out on a small area.

Thank you for helping.

Lots of people use any breathable paint for diy places if you are using in on a listed or old building. Their are other people who use a through colour render or plaster, which means is more cost effective throughout the life of the building as it doesnt need painting every so often. Regarding the lime and cement why are you mixing these together. Rule of thumb is never mix the two. if lime was used then lime is required and cement is used then use cement. All this will do is cause salt and damp issues down the road and decay of the building. Anyway, these guys have through colours and highly accredited
limetec.co.uk/lime-render/