Wall stains in chapel

[Jameswren]

I’m after some advice on some staining on walls.

The stains are on an internal wall in a renovated chapel that is damp internally and very wet at low levels. This is a known issue we did not attempt to rectify.

The building’s construction is a ragstone outer skin, brick inner skin and a loose fill or general rubble, mortar, ragstone etc as an infill between brick and ragstone. We did no alterations to the structure but did remove the loose plaster then apply a sand and cement layer.

The loose plaster was removed, replaced with sand and cement then painted using Dulux Fast Matt (that was recommended by Dulux). We left the sand/cement around a week to dry and used dehumidifiers, although the room was probably damp at the point we painted it.

Checking the work around 6 months later, there is a light grayish stain running exactly where the joint between the old and new wall finishes meet.

I would suggest this is water ingress but why would it only appear at the joint between old and new plaster? That therefore makes me think it is something to do with the plastering/decorating.

 

[dcdec]

Different types of material used in plastering i expect, ones more porous than the other

 

[Jameswren]

Possibly but why would it only appear at the joint between old and new plaster? If the issue is how porous one of the finishes is, I'd expect the whole of the new surface to be patchy, not just at the joint.

Somebody else has suggested some form of chemical reaction at the joint - either the paint reacting with the lime/cement or the lime reacting with the cement.

Dulux have said we didn't leave the render long enough to dry but again, why would it then only be at the joint?

I suppose the only way to find out is to re-plaster a section or to stain block and re-paint, to hope it clears the issue.

 

[Nige F]

Have a look @ Classidur paints - I remember we used it once on a Church job - you won`t be Crying in the Chapel with that stuff