Steam stripper (wallpaper) causing plaster to blow

[Phil Anthropist]

I have stripped wallpaper using my steam stripper loads of times but today I have been doing a neighbour's room and had to stop after several areas of the skim coat cracked and bulged forwards. Presumably there was air under the skim coat and the heat from the steam caused the air to expand and blow the plaster. The house looks about 30 years old. Was this due to poor plaster or plastering? I didn't leave the stripper plate on one spot for too long, and some of the plaster cracked after only a few seconds of steam.

[Andrew Gabriel]

Was the wall damp? The stripper will cause moisture in the wall to boil, which might blow the plaster.

[Phil Anthropist]

No, dry as a bone, the lowest areas are at knee height but most of the blown areas are at chest, shoulders and head height (I am 6'2"), which I assume would exclude rising damp. The affected walls are internal walls in the ground floor stairwell and entrance hall which I doubt would be subject to condensation, and I can't see any evidence of damp from any other source such as a leaking pipe. The blown areas vary in size from a saucer to a dinner plate, some larger areas are about the size of 3 or 4 saucers. I got the impression that the lady who owns this house anticipated there might be problems; she didn't seem surprised by what happened, and she wasn't in the least curious as to why. Strange!