It's called Bridging in the trade! Allowing water or moisture to travel from one point to another.Soot contains high levels of salts that are hygroscopic. They will absorb moisture from the air, which will then travel through the dabs to the plastered surface and produce a brown/yellow stain. Just the same as when there's been rising damp, but different salts.
Have you fully removed the chimney or put gallows brackets inhi all
i've removed a chimney breast upstairs and down and have soot deposits on the walls. im dotting and dabbing the whole house so want to do the same on these walls, because i can do it quite well and i'm terrible at plastering otherwise and it's cheaper for me to D&D and then have it skimmed then have a plasterer do it all. View attachment 49929 View attachment 49930
i will mechanically remove as much of the soot as possible and then...
ive read up quite a bit on line, both on this site and others and it seems my options are:
1. batten the wall - i don't want to do this as the bricks are hollow and drilling and fixing is a nightmare.
2. D&D Gypsum MF10 metal channels to the wall and screw board to these
3. render the area +300mm either side and then D&D over this
the plasterer that will be skimming to me to get some soot inhibitor, but no idea what he's talking about and can't find anything of this description online.
the MF10 channel would be easy to do but a bit pricey at £50 for 10 lengths.
where it gets tricky for me is the various suggestions for rendering i've read and knowing which i should do. coats of SBR first, slurry or no slurry, render with SBR in, Render with lime in, one coat of render or two etc. i'm a bit at sea with it all! and there are so many products out there that seem to do the same thing in terms of tanking or waterproofing.
can anyone give me some clear advice on the steps i would need to take to seal the wall via rendering prior to D&D to stop damp and staining coming through? the surface is very uneven where the tied-in bricks have been cut through, as per photos. It's a party wall, if this is relevant. the whole wall on each floor is 2.5m high and 3.1m wide. the chimney breast was 1.1m wide.
thanks in advance.