Why is Skim Coat blistering/bubbling ???

[Stu1H]

I have today patched up an area of blown plaster and the top coat (multi finish) started to bubble up in a few areas. Can anyone tell me why this is happening?

The area is approx 1m square, and repaired as follows:-

- Brickwork primed with PVA 1:3 a few days ago.
- Last night heating in kitchen turned off.
- Brickwork PVA'd 3:1, 1st coat of bonding (about 8mm thick), surface scored.
- 2.5-3 hrs later, 2nd coat of bonding (about 8-10mm thick), surface scored again.
- 2.5-3 hrs later, multi finish (3mm thick).

The bubbling wasn't confined to just one area but appeared at random (a total of about 10-12). The only way I could seem to get around it was to score through the bubbling with a stanley blade.

I've got to do another area tomorrow of about 3-4m square and obviously want to avoid this happening again.

Any help/advice appreciated. Thanks.

 

[roughcaster]

Read this from Noseall and Joe,

//www.diynot.com/forums/viewtopic.php?p=296330#296330

Roughcaster.

 

[trowelmonkey1]

leave it longer before flattening it, messing with it will make it worse as the link to the post above will show you.

PVAing the wall has slowed down the set of the bonding and water from it is coming out from the surface of it rather than being sucked into the wall i think.

so on your next patch, leave it a while longer before skimming and if bubbles make a come back, leave it as long as you dare before flattening it, then leave it a little while longer before topping it.

let us know how you get on

 

[Stu1H]

Cheers guys. The larger area went fairly well and no probs with the bubbling up. I done the first coat of bonding in the afternoon, 2nd coat quite late at night and skimmed first thing in the morning!!

A few bits need filling up but for a first timer i'm chuffed

However .... Another issue is worrying me, on the first area I done I've noticed a very fine white powder/cyrstals forming on the edges of the old plaster where it meets the new. It's not all the way round just in a couple of places. Anyone got any ideas what could be causing this?? I got a guy coming round after the weekend to quote for skimming my lounge ceiling i've just boarded so will see what he thinks.

 

[roughcaster]

Sounds like it could be a little efflorescence/salts coming through the new plaster from the wall.

Roughcaster.