I,m just in the process of refurbish an old Victorian cottage, on stripping the wallpaper in one room, the seraphite skim behind came off with it on the top half of the walls. It looks like historically the wall (which was an external rubble stone wall but is now an internal wall due to an extension) suffered problems with damp and somebody gas tried to remedy it by removing the lime plaster on the bottom half and re rendering with sand cement and then applied a bitumen paint to the render,and then skimmed the whole wall with a gypsum plaster (the top half over lime plaster, the bottom over the sand cement) however the skin has not taken to the lime plaster – so I now gave a wall where the bottom half us a skim finish and the top half a lime plaster finish, with a 4mm step half way up. Do you have a recommendation on resolving this? As I see it I have 2 options: –
1. Remove the lime plaster top half, render the top half with sand cement and re skim.
2. Remove the sand cement bottom half, re render with a line product and then apply a new lime top coat to the whole wall.
Also the other lime walls have a very poor but sound(no hollows or loose patches), would it be okay to apply a lime top coat over this?
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We would recommend option 2, firstly as the cement is likely harder than the host stone and this could cause damage to the substrate and secondly if any moisture is present, lime will allow this to transfer through the wall.
With the second question it is more difficult to answer without seeing the walls in questions, but providing the background is solid, free from paint, is porous and has a suitable key, a lime plaster can be applied over the top. It would be recommended to start spraying the wall down with water in advance to help control the suction. You are welcome to email some images through to firstname.lastname@example.org so we can offer further guidance.