Sandstone walls, concrete render removed (ish)

[zunetunes]

We had one very wet wall due to penetrating damp. Have stripped the several layers of plaster which were on top of a concrete coat. The other walls in the room are lime plastered so assume this one was lime originally. We would like to leave the wall as bare stone (which will be needed for some time more anyway while it dries off).

The problem is that although the concrete layer has come off, the stones are still covered with a fine layer of something, which I can't get off, leaving all the stones an off white colour. I don't know if this is the dust from the original lime plaster which has somehow bonded to the stones when the concrete ws put on. Have tried chipping away at it and using a wire brush. Where there are random bricks in the wall the brush cleans them up well, but it hardly touches the stones. Any suggestions?

 

[MdB]

What type of stone is it? How porous is it? Could the cement have gotten into the surface? My stones are very very porous but the cement didn't penetrate as I think it just flash dried on contact so I was lucky and it came off cleanly.

Any chance of some pics?

 

[BMcD]

Have you tried cleaning the surface with water and a carborundum stone?
It might be worth giving it a go in an unnoticeable area first to see how it goes.

 

[zunetunes]

Mainly sandstone

 

[geoff]

Could it be hardened PVA with some cement/mortar residue in it? It's been common for some time to slap on a coat of PVA before applying a concrete render, especially on crumbly or damp walls. It can be a bugger to remove if it is.

 

[Lime]

Could it be hardened PVA with some cement/mortar residue in it? It's been common for some time to slap on a coat of PVA before applying a concrete render, especially on crumbly or damp walls. It can be a bugger to remove if it is.

I suspect that is what the problem is.
It is very difficult to remove.
sometimes it can be washed off depending on it's consistency and age.
I wouldn't use a carborundum stone as it will mark the sandstone.
Sometimes a steel or brass wire brush with water will remove it.

 

[zunetunes]

Thanks for replies - haven't tries the carborundum, have tried wire brush - it gets a certain amount off but then no more - it also turns the stones grey!

 

[Lime]

Use a water spray.

 

[Gervase]

PVA's a swine to get off - I've found the best way involves a bucket of water (changed frequently) and a green Scotchbrite-type scouring pad (The Tesco 'value' range ones in packs of three for pennies are ideal - and also good for cleaning trowels and hawks). Wear rubber gloves and be prepared for a bit of mess.

 

[chuckey]

Try water and a stiff brush first (for loose stuff), then brick cleaner (for cement/lime mortar), then boiling water/steam (for PVA). This system will work and not marr the surface at all. Sandstone will be marked by wire brushing. Its a tedious job
frank

 

[zunetunes]

Will try brick cleaner - won't damage the lime mortar? Is there a proprietary brand you can recommend?

 

[Anne]

This might seem like a stupid question but are you sure it isn't old limewash left on the stones?

 

[chuckey]

The acid in the brick cleaner disolves lime and cement, but the colouring you see is only .002" thick, I doubt if you will detect this amount of surface loss on your pointing. When cleaning a proper mortar of stone, you will get an initial "fizz", its the surfce lime/cement being disolved. What is now left is a surface which is predominately sand particles which are shieldinng the rest of the cement. This surface is then removed mechanically (stiff/wire brush), cos the bedding of the particle has been weakened. More acid, more brushing. . . The only thing It won't touch are some of the "limewashes"/ancient paint, I think they used casein / linseed oil as binders.
Frank

 

[zunetunes]

Thanks - I'll try that