It can be that hollow sounding plaster is absolutely nothing to worry about, as patches can lose their bond and be hollow very soon after application. If these areas are not that great an area, and anchored every now and again by adhered plaster, then there is often no problem.

Can you fix blown plaster?

No way to repair, as above, chop out the blown bits. Mix a solution of PVA, apply to the substrate and especially to the edge between plaster and wall. This will help to bond the new plaster and also help to avoid any further blow off.

Should I remove 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.