stains appearing on skimmed and painted walls

Hello, Id firstly check that theres no damp coming through the walls first. I would of said stain block walls then re-coat as normal but as youve done this there seems to be an underlying problem. If there is no damp try using a stain block called zinseer, quite expensive but its bullet proof to stains. Then re-coat using any paint finish you want. See now that goes and if stains do come through theres definatly something happening on that wall which might need hacking off and inspecting by a proffessional. Hope it helps. In relation to your reply, was the walls dot and dabbed or was they fixed to a stud? Are the stains uniformed in shape and size ? If they are then it could be the plasterboard adhesive (dabs) drawing moisture from the bricks then penetrating through the board. As for insulation was it cavity filled or was it on the internal side of the wall? Even if you didnt mist coat and you painted the walls wet then i cant see how this would produce stains as it would of all dried together eventually...? Plus youve stain blocked it- Im not saying its ok to do that as its not good practice for newly skimmed walls but i cant seem to draw a conclusion to the two. Thanks

Sounds to me like the galvanised clout nails used to fix the boards are starting to oxidise. Are they appearing in straight lines? Anyways. Stain block, aluminium primer or oil based paint, however, as all these have a tendency to shine through emulsion paint I would also recommend lining paper.