Skimmed walls - small hollow patches

[houseonahill]

1950s house, old skim was in a bad way so got builder to remove all the loose stuff. Some quite large patches were hacked off and we could see places that had been filled and sanded over by someone in the past.

Walls were then thistle bonded and plastered over. Was allowed to dry without heat on for a couple of weeks. I've just gone round the walls tapping to check them and there are quite a few small spots here and there where it sounds hollow These patches are not very large, on average about the size of a beer mat or smaller. I'd say there are about 20 or so such spots in one room, in random places.

The new skim feels as though it has swollen ever so slightly in these places but it also feels strong and does not move when pushed, nor is there any cracking except for in once place where we had some bricks replaced behind - just some hairlines over the mortar joints.

By the way, the plaster went on more than six weeks ago now. Heat has been on regularly during the last three weeks.

My guess is the old skim, once moistened when the new skim was applied, delaminated in a few places.

I know that one option is to hack the whole lot off, go back to brick and start from scratch but I would like to know if people feel this is really necessary. We spent a good bit on the skim and just don't have the budget for more right now, though I suppose it's something we could do some years from now if the new skim turns out to be unstable.

 

[Tigercubrider]

I have repaired similar by drilling a small hole and filling a syringe with unibond.
Squeezed the unibond into the hole and it works

 

[houseonahill]

Thanks @Tigercubrider but these patches are pretty small and the separation from the sand cement coat must be 1mm or less - I would fear that injecting anything could just prize the skim away from the wall even more! Also not sure it would dry quickly enough... Wouldn't it be better to just cut out the patches with a stanley knife and fill / sand?

 

[Deleted member 292770]

why bother? It’s not cracked or moving you said. sounds like you’re worrying about virtually nothing.

 

[houseonahill]

Hi @Godwasaplasterer, I suppose the sight of the old skim crumbling in front of me has made me nervous! The new skim is not problematic right now, that's true, I suppose my only thought was whether it would get worse over time and eventually crack / flake away. But if the chances of this are low / it would likely be many years before this happened, then of course I'd be happy - and somewhat relieved - to just leave it.

 

[Deleted member 292770]

there’s potential for all skim coats to crack over time with movement and expansion and contraction. as for modern gypsum to ‘crumble’ that’s unlikely.
I’m assuming your plasterer pva’d the wall before skimming?

 

[houseonahill]

No @Godwasaplasterer it was just Febond blue grit - realise I said thistle bond above, sorry!

 

[Deleted member 292770]

that’s better actually. but I wouldn’t have relied solely on it on the bonding coated areas. on your bonding coated areas it wont have controlled suction as well or given as much of a bond as pva .
You could trim the areas out with a blade as you suggested earlier , but that comes with the risk that you may discover there’s a lack of key and adhesion everywhere and you’ll be able to take more and more plaster off.
sorry , don’t mean to be depressing, seen it a hundred times.
i’d mist coat it and go from there.

 

[houseonahill]

@Godwasaplasterer When you say "bonding coated areas" - do you mean everywhere that had blue grit (which was the entirety of the walls) or just the areas where old skim was still present?

 

[Deleted member 292770]

where the patched bonding coated areas are , i’d have pva’d.

 

[houseonahill]

@Godwasaplasterer ??? I don't follow - the walls had areas of old skim removed because it was loose (about 35%, in random places). The rest of the old skim was firm and remained in place. Then, the whole of the walls, top to bottom, was covered in blue grit. And the spreader then put new skim across the entirety of the walls.

 

[Deleted member 292770]

Sorry the bonding bit threw me.... there was enough key on your walls , blue grit was the wrong option. i’d have pva’d to control suction .