Can anyone identify this material please ?

[OatsAndMilk21]

Hi there, I had a leak from upstairs, which is now sorted, so decided to remove the stained wallpaper from the top of the wall and ceiling as some mould had formed, just in that area. The house is from the 1930s and in good condition. What am I dealing with here ? Is it concrete ? It has a few cracks, but the area I uncovered is 140x50 cm and in one piece. I need to know what it is to possibly seal or prime it / paint it. Would anybody be familiar with that material ? It is as hard as stone, porous but not too rough. I understand now why it was wallpapered.

 

[JohnD]

I expect it's lime plaster, though the appearance is not what I'm used to. Chip a bit off, drop it in vinegar. Does it fizz? Is grit left behind?

It might be sand and cement render, and probably had a thin coat of hard finish plaster which seems to be missing

 

[foxhole]

May have clay or sand content which can change the look of lime plaster, depends what was available locally at time of construction .

 

[opps]

As a decorator, my advice would be to sand the filler on the ceiling (as much as possible). If you sand too much, the grit in the finish will rip the surface of the plaster.

To be honest the existing filler looks pretty awful. Perhaps use a chisel to remove the obvious snots.

Yeah, it is probably lime based.

If you can't safely sand it back, apply a coat of SBR and then use something like Toupret TX110 filler. Then patch in a layer of lining paper.

If you use the correct fillers, the repair should be pretty invisible, At minimum you may need 80 grit paper and then 180 grit (where the new lining paper meets the old). If the edge of the old paper looks like it may lift, brush in some PVA glue (or wallpaper paste). Then apply wallpaper paste to the plaster, apply the paper (with a coat of paste on it). Once dry, apply a coat of emulsion. It will help to seal the meeting edges, shellac paint would be better... but.

 

[OatsAndMilk21]

Thank you both for your replies. I have done the vinegar test - it did bubble slowly for a long time and left grit behind but the exterior layer of it was left in shape. I also found some horse hair in the wall behind the piece I took out.
So as it sounds like it is lime plaster, I have been reading about it. It says that only "breathable" paint should be used. I was going to use some caulk in places and also some stain blocker on the slightly darker area where the mould was. Somehow I doubt that the paint on the wallpaper was special paint, this room was redecorated by the previous owners around 2005. Would I cause dampness problems by using water-based emulsion ?

As a decorator, my advice would be to sand the filler on the ceiling (as much as possible). If you sand too much, the grit in the finish will rip the surface of the plaster.

To be honest the existing filler looks pretty awful. Perhaps use a chisel to remove the obvious snots.

Yeah, it is probably lime based.

If you can't safely sand it back, apply a coat of SBR and then use something like Toupret TX110 filler. Then patch in a layer of lining paper.

If you use the correct fillers, the repair should be pretty invisible, At minimum you may need 80 grit paper and then 180 grit (where the new lining paper meets the old). If the edge of the old paper looks like it may lift, brush in some PVA glue (or wallpaper paste). Then apply wallpaper paste to the plaster, apply the paper (with a coat of paste on it). Once dry, apply a coat of emulsion. It will help to seal the meeting edges, shellac paint would be better... but.

I am trying to stay away from lining paper but the difference with the new patch might not turn out pretty...

 

[opps]

So as it sounds like it is lime plaster, I have been reading about it. It says that only "breathable" paint should be used. I was going to use some caulk in places and also some stain blocker on the slightly darker area where the mould was. Somehow I doubt that the paint on the wallpaper was special paint, this room was redecorated by the previous owners around 2005. Would I cause dampness problems by using water-based emulsion ?

I am trying to stay away from lining paper but the difference with the new patch might not turn out pretty...

There are occasions when lime plaster needs breathable paint.

I primarily work in victorian houses in west London- 9" solid walls with lime plaster. None of them have breathable paints on the walls.

Until the leak, you had no damp issues even though you had non-breathable wall paper paste and non-breathable vinyl matt emulsion.

BTW, if you don't re-line the wall, I very much doubt that you will be happy with the finish...

 

[OatsAndMilk21]

Thanks to all of you - I am listening to your good advice and going the re-lining way with emulsion!