Ummmm's filler...

[heathfield]

Very strange. Typed an answer to Ummmm's post, clicked on Submit, and the answer disappeared into the ether. Twice more - same result. What's up, Ed?? Anyway...

Dilute 1 part PVA with 5 parts water and apply with a paint brush to soak the plaster in and around the holes. Let everything dry thoroughly. The plaster won't be powdery any more. Fill the holes with Polyfilla or a similar DIY filler.