When damp seeps through your plaster it can cause it to deteriorate over time. This can cause the plaster to blow, crack and crumble. The main reason for plaster blowing, is that it has lost adhesion to the surface it was applied to.

What does it mean when plaster has blown?

Plaster blows because it either has no, or has lost its, adhesion with the surface it has been applied to. This can be for a variety of reasons the most common of which are 1. The surface its being applied to is very porous.

Can you save blown plaster?

Yes............ carefully remove the loose/blown areas (don't go mad as you don't want to disturb solid areas to much) treat bricks and edges of existing plaster with some PVA/water mix then re-plaster with Thistle Hardwall.