Damp mark showing through new paint

[suzemuze]

We've recently had our living/dining area completely knocked through to make the space open plan. The structure that was removed between the two rooms wasn't load bearing and was more of a 'feature' as there was already an rsj in place.

When it came out some of the remaining wall near the ceiling got damaged, so our builder repaired it with cement (I think) and replastered (the plaster work was pretty deep so took a while to dry). As it was drying out, we started to see some salt appearing on the surface. As this was near the ceiling we thought it probably wasn't a damp problem and just that the plaster was so deep and taking its time to dry. However we have recently had a leak through the ceiling (on the other side of the room) so I am now wondering if this has possibly caused it.

Anyway we brushed off the salts and they didn't reappear and the plaster appeared to dry completely. We primed and painted - but today I can see a mark has appeared (where the salt was) - like a damp stain. Basically on the edge of where the new plasterwork met the old wall.

I'm not sure what the best course of action is? It's an internal wall, as far as I know there's no existing leak and it's near the ceiling so I can't think why we have this problem. The paintwork isn't blistering or anything - I don't know whether we should paint the mark with damp proof paint before we apply the second coat, or whether we should apply a vinegar solution which I've seen on lots of forums to be a salt neutraliser.. However we don't have salt deposits anymore - just a mark on the wall ruining our lovely new paint!

Any help/advice would be greatly appreciated

 

[dcdec]

Sounds like its just a water stain, standard emulsions won't block it. You can use zinnser cover stain or some thinned (with white spirit) oil based undercoat. The builder probably used a bonding coat before the finish plaster and this always takes longer to dry, the salts are pushed to the surface by the water drying, you did the right thing by brushing them off and are unlikely to reappear unless the plaster ever gets wet again.