We had these blown patches of plaster on an interior wall when we first moved into our home. We are now wanting to decorate for a new baby, so we have knocked the old plaster back to where it was not hollow and wanted to fill it in. The thing is we can see this very dark patch which isn't plaster it is a metal and it is just rubbing away. It smells very metallic and will just stain through the plaster. This is not the only patch like this, there are 3 and also hollow patches in other bedrooms which I wonder if it is the same under. We had a man round who told us everything was condensation. We live in a flat so it is expected but we do everything to keep it minimal. We have a leaking gutter outside just by the wall in the picture but he said homes like ours don't suffer from water getting in. (Though we have a leak round our bedroom windows in which 2 floors up and above where suffering from as well due to the horrific weather back in December..but again we were told it was condensation) and a leaking drain by our kitchen. We have asked for all this to be repaired.. now would condensation cause such blows in the plaster and cause whatever the metal behind is to corrode? How would you go about making it so the metal doesn't come through the plasterwork, or does some work need doing here to replace whats there? Thank you for any replies and help