Trouble with high suction patches

[Harrybrice]

The plaster has blown on different patches on some of the walls in my house, under the plaster is old, and very very dry sand & cement render. I have tried doing 3+ coats of PVA on these areas but when skimming over them they dry up the plaster near instantly.

What do you recommend for things like this? I was thinking that something like bitument paint would seal these high suction patches off before skimming, I'd rather not buy a 5L tub of SBR/bluegrit if I don't need to.

Could it work to just bond these patches a short while before skimming?

 

[Stewie03]

The key on the old render has come to it’s end there’s only one thing for it HACK IT OFF, HACK IT OFF, WANK IT OFF WTF HACK IT OFF

 

[MakeItSmooth]

Hi smoother