How to skim over this old wall with some blown plaster?

[houseonahill]

I've seen lots of threads about this sort of issue but every case has details that are slightly different to ours... and I want to get the right advice here.

We are renovating our 1950s house and all our walls are like this (see below). Previously, all walls had wallpaper - which was carefully removed - but lots of the old skim came away very easily.

Some of the old skim is stuck fast and sounds solid when knocked but I'd say around 20% is hollow / loose or has already fallen off. The walls are brick (red brick / cinder block) covered in what I guess is a traditional sand cement coat. We don't have lathes.

My question is this - if you were going to skim over this, how would you prepare the wall to ensure good adhesion and a level finish? PVA? FeBond Blue Grit? Wet it down first?

Can someone write out the exact steps one should follow so I know. I am getting a professional to do this but just want to know the ideal approach before they dive in.

 

[Wayners]

I'd chip the wall back to the backing plaster. You don't take off all the skim coat. By doing this any loose bits will come off and then any sound plaster with have the chips dug through for the new plaster to stick. If that makes sense.
I'd give it a coat of beeline sealer following guidelines as that kills the suction.
Then you have options.
Bluegrit some like as nice to skim over, but I tend to PVA and when sticky plaster. Just quicker and easier for me.
Bluegrit is like beeline ( polymer) but has sand in it so using beeline first makes it easier to apply bluegrit plus the extra sealing of the surface.

It doesn't matter really what you use over products like beeline tbh. It's just what you prefer.
PVA on its own works but I struggle with suction and fast setting of new plaster.

I've also used gardz to seal then PVA over.

So to answer.
Chip through surface skim. Don't be too fussy with it.
Beeline or gardz. Allow to dry.
PVA and skim

 

[houseonahill]

Thanks @Wayners - if I understand you correctly, you mean take a hammer and chisel and chip at regular intervals / rows along entire walls. What's loose comes off, what's not stays on. Then prep + skim?

(The other option I have is to board it all then skim - but I'm wary of that due to losing space, higher cost and any risk of damp / mould behind boards and so on)

 

[Wayners]

Thanks @Wayners - if I understand you correctly, you mean take a hammer and chisel and chip at regular intervals / rows along entire walls. What's loose comes off, what's not stays on. Then prep + skim?

(The other option I have is to board it all then skim - but I'm wary of that due to losing space, higher cost and any risk of damp / mould behind boards and so on)

Yes you have that correct.

No don't board over