Hi all, I recently had a new chemical DPC injected into the end wall of my 1930's end terrace. The contractor removed the recommended metre of internal plaster prior to injecting the DPC and I've re-plastered using Thistle dri-coat. I now want to skim the whole wall but one of the chimney breasts is already skimmed using a mystery (to me) black crystalline material. The attached pic shows a hole in the plaster where there was a picture hook and I've run over the entire wall with a sander hence the black dots surrounding the hole, where the sander took of the tops of the stipple effect. Going in from the surface, the materials seem to be: paint, the mystery black crystalline material, skim, bonding coat and wall. As this is a chimney breast, could this material be some sort of thermal barrier or DPC? Strangely the other chimney breast doesn't have this material, just skim and woodchip. Any thoughts would be welcome and yes, fortunately I wore a dust mask prior to sanding and I painted over with a strong mix of PVA immediately after. Thanks.
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