New plaster sounds hollow!

[Lexi]

Hi,

I recently did a skim coat of finishing plaster over the old plaster in our hallway. I removed all of the old plaster, sealed the wall with diluted PVA and left to dry. Then before applying the skim coat I covered the wall again in a coat of diluted PVA (about 50/50 water & PVA) then applied the plaster while this was still wet/tacky. The plaster seemed ok and did not go off too quickly, but now weeks later i've noticed that if you knock on some areas it sounds very hollow like it's not adhered to the wall behind. Does anyone know why this might be and what i've done wrong??? I've got to skim all the bedrooms yet so I don't want it all to fall off!!!!

Thanks!

 

[Richard C]

A bit confused; you say you removed all of the old plaster but then say you did a skim! Did you remove both the finishing plaster & the base coat down to bare blockwork or was there a secondary skim already there which you removed?

If you’ve just skimmed over bare blockwork, that's your problem but if you’ve skimmed over original plaster & it sounds hollow then the original was already ‘blown’ & no amount of skimming over that will put it right. You have to remove everything back to bare blockwork & start again; I’ve had to remove 70% of the plaster in my 60’s house renovation because of this!

 

[markthebuilder]

what sort of plaster are youusing

 

[Lexi]

Sorry i didn't explain very well!!

We removed the wallpaper and left the original plaster, we just removed the bits that were blown (only a few small patches) and left the sound stuff.

We then sealed all of that with diluted PVA, then when that had dried we filled in the holes with thistle hardwall plaster,, then levelled off.

Then when that had gone off we went over the whole wall again with more PVA then with a thin skim of Thistle multi-finish plaster while the PVA was tacky. there seems to be only little bits here and there where it sounds hollow and it's only on the areas where we left the original plaster.

 

[Richard C]

You seem to have done the skim correctly; assuming it's not a stud wall or been dry-lined!! the hollow sounding bits you’re now discovering are almost certainly where the original base plaster has delaminated from the blockwork. It was probably there before you re-skimmed but it sounds as though you didn’t spot it & only repaired the areas where it was obvious.

I’ve got exactly the same scenario in virtually every room in my 60’s house & I go over the entire wall tapping & marking to see how bad the problem is. Some walls where there wasn’t much worth saving, I’ve had to strip it right back to the blockwork & start again; others, where there was only the odd hollow sounding, it didn’t justify stripping it all off so I’ve done local repairs, as you’ve have, reinforced the ‘join’ with fibre glass mesh tape (helps stop crack lines developing around the repair later) & then re-skimmed the whole wall.

Short of stripping it all off & starting again, there’s not much you can do really! Personally, if it looks OK, I’d leave it alone & see what happens; maybe it’s been like that for years & will stay there for many more yet, you can always start again if it does begin to fall off in the future.

 

[Lexi]

thanks for your reply Richard, it's definitely eased my mind. the bits that sound hollow don't move so i think i'll leave them as you suggested and see what happens, i can patch up any bits if they fall off! that's a good suggestion about the mesh tape, i'd not thought of that. i've got a number of rooms to patch up still so i'll use that on the next room i do!
thanks again!