Hi Have just moved in to new house which is a 1920's semi.Wife decided that the hall needs redecorating and promptly starts emulsioning the ceiling.Shortly after she calls me to have a look at what looked like at first condensation on the wall but on touching found it to be a greasy/oily,colourless substance with a faint fishy odour.Tried wiping some off with tissue but looks like it covers a sizable portion of the wall.The wall is bare plaster emulsioned and is the exterior facing wall which has a damp proof course.Could anyone shed any light on this - would appreciate it - Thanks.