PVA or not onto Bonding?

[NickB_99]

Hello all,

After a spot of advice on the use of diluted PVA or not when using Bonding...

I've patched up some interior walls with bonding coat in weak areas (area-wise about 10% of the wall). The remainder of the wall is emulsioned paint over plaster (prob about 20 yrs old).

I PVA-ed the painted part before skimming with quite a strong mix of PVA water (about 3:1). The adhesion and setting of the skim was fine.

Where I've struggled is with the bonded areas. I was taught that when using bonding, if you PVA over it (the case in hand was within 24 hours) that it would restrict the moisture coming out of the bonding and could cause the skim to bubble.
However, where I have bonding I have not been so efficient getting back to it, so it had dried out fully to a light pink.
I applied water only and then before skimming I applied a second pass of water to try to reduce suction out the skim. Hoped this would do the trick - it didn't quite!
The first coat over the bonding areas dried out fast and crazed a bit. It didn't lose adhesion, and fortunately on the second coat covered just about - but didn't allow much scope for trowelling, as had gone off quite significantly compared to the adjacent skim over the painted areas.

Apologies for the essay, and assuming you've not fallen asleep - should you just use PVA/water mix as normal and treat it all the same once dried out fully?

From the posts I found on this subject, consensus seemed skim promptly after bonding, as moist bonding is easier. And PVA isn't quite the no-no I thought.

Advice appreciated as always.

 

[skinters]

dont quite understand what you mean by using pva over the bonding,pva is the bonding eh ?never used bonding coat myself .

in my experience some backgrounds need a few coats of bonding.apply the pva and leave for five min,if it goes dry quickly,apply another coat.idealy the pva should be tacky to the touch before applying the plaster.

personaly i would never apply plaster directly on to wet pva.

 

[roy c]

If the bonding has dried out completely then I would coat it with unibond (PVA) 3-1 is ok. Leave it for 24 hours and before you skim it apply another coat of unibond mix up your "gear" and skim as normal when unibond is "tacky",if you wanted to you could "mist" spray the bonding with water until it absorbs no more water (which means you have killed the suction) and skim as usual...I have skimmed on unibond when it has been wet, with no adverse effects!!! Good Luck......

 

[skinters]

hi roy

concerning plastering straight on top of wet pva,i have found on certain backgrounds,the plaster reacts funny and comes away from the wall.strange it is .be interested if anyone has had similar happen.

 

[roy c]

Hi skinters,
I understand what your saying,but what I meant is if I have prepped the wall previously (24 hours before) and then put the unibond on and skimmed when the unibond is still wet, the plaster still goes on ok, sometimes it mixes in with the first coat but when left and second coated, its fine. I'm sorry I didn't mean it to sound like it did when read back..

 

[NickB_99]

Thanks guys. Will give it a go.

 

[noseall]

agree with the guys, once any gypsum backing coat has been allowed to fully dry out there is little alternative but to damp down with diluted pva.

a weak pva mix is all you need and can skim up straight away. after all you are only re hydrating with a little help from the pva. the pva will skin up almost immediately so you can add a second coat just prior to applying the skim.

 

[NickB_99]

Was going to put up new a post - but realised there was a big overlap from an earlier one I put up.....

If covering Bonding with Multi-finish, best case is skim when the Bonding has darkened (same day) - no PVA needed.
If it has completely dried out (e.g. left 1 month+), diluted PVA is fine.

Ok, the new bit....
My dilema is - I have been plastering some areas with Bonding and not got back to it until the next day, or even a few days later. Moisture is clearly still there as it has not gone to a very light pink.
Before skimming, I wanted to do something to slow the suction down.
I first tried just applying water, but even next day the Bonding had such a thirst
So I've used a very dilute PVA/water (prob 1 to 7 ratio, so is very thin). I tried not to drown every square inch, but give a reasonable cover.
All has been well so far.... Multi went on fine and no blisters or anything developed etc.
The question is, will this cause problems in the long run? Does applying even a very dilute PVA cause it to seal and waterproof (thus preventing moisture coming back through)?
or does it just slow down the moisture release?

Any tips on the ways of the PVA appreciated!