Dark spots in hardwall.

FormerGreat

Hi,

Just wondering if anyone has came across the same condition as described below and any ideas on a solution.
A cavity block with insulation extension was added to house over two years ago. To increase insulation we used 50mm insulation backed plasterboard on new extension and existing solid stone walls. The builder used the metal mushrooms to fix the plasterboards and a skim coat of hardwall.
I gave room a few coats of paint and everything looked ok. However, soon after dark spots have appeared in both rooms ( cavity and stone wall) mostly nearly the top of walls ( sometimes not) where I presume the metal mushrooms were used as the spots are the same size as head of these.
Now I've heard that we could use a paint primer to seal the surface and that would solve issue but I'm not sure.
Any ideas appreciated. Am trying to get builder back to view as it can't have been the first time he has used this fixing method.

Thanks,
FormerGreat.

mad m

Think you have cold bridging with the metal fixings....Afaik, you were only meant to use plastic mushroom fixings and two metal fixings (Fire Regs) to hold up an Insulated board.