Damp walls, failed damp course - quick fix needed

[hamerr]

Hi,

Prior moving into my house a damp course was installed, evidently by cowboys, who have been back but said I needed to coat in a vinegar type solution and something similar to PVA. I did all this and having wallpapered there's a patch showing. Interestingly the damp co also did the survey for the house and said everything was okay; which evidently it wasn't; but that's another story.

I don't want to strip the paper off so if I coat the paper with either an oil-based undercoat or damp seal, then emulsion over, will it show through?

 

[Johnmelad]

What sort of damp course?

 

[hamerr]

Not entirely sure but there's holes been drilled on the outside of the wall and some kind of mortar used on the walls on the inside.