Plastering on blown substrate

[derekphl]

I've been working on a room where I removed the chimney breast and various bits and pieces.

Having attached insulation backed board to one wall, over-boarded the ceiling and stripped the wallpaper off the remaining walls I have now started to plaster skim each surface.

Yesterday I did my first wall (one stripped of wallpaper) and despite a few necessary newbie mistakes and imperfections, I'm pretty pleased with the outcome.

Today I started on the opposite wall and it didn't go as well. I had PVA'd the wall with a very thick mix and left overnight, but several areas where the plaster substrate was evidently blown, cracked within an hour of applying the second coat. Oh well.

1. The blown areas are otherwise flat and sound, can I get away with a couple extra coats of unmixed PVA, or do I need to go the route of pulling off the blown plaster then hardwall etc?

2. The third stripped wall is fairly good, but does have a couple of dodgy areas, so is there anyway to avoid pulling all that out as well?

Thanks in advance.

 

[Doggit]

You need to knock off all of the blown stuff, and then stabilise the wall (with something like this) before staring again. PVA will stop the wall sucking out the moisture too quickly, but that's all.

 

[derekphl]

Not come across that stuff before, interesting.

Would I get away with just extra PVA on the smaller areas? They're not much more than a few inches wide in most places.

 

[Doggit]

If it's only that size, then you might. The stabiliser binds the wall together, but you always need to hack off as much of the loose material as possible, otherwise it might carry on cracking afterwards.

 

[derekphl]

I have to say, I'm not sold on the idea of using stabiliser for this wall. What generally seems to be talked about for using this stuff is a) that the wall is exterior and b) it's going to be painted - neither of which is the case here.

I reckon hardwall is the consensus in these situations, i.e. a surface that is not low-suction (old brick).

Am I wrong?

 

[Doggit]

One possible mistake that I didn't spot earlier, is that you'd used unmixed pva, and that won't adhere very well, as it can't soak in to the substrate, but stabiliser does a different job, in that it binds the wall, so you have a type of stabiliser that seals in dust, and a type (often for outside walls) seals in a bit deeper, so it's doing a similar job to PVA, but a lot more thoroughly. But as I said earlier, then main use of PVA (1 part pva to 3 parts water) is to reduce the suction on the wall so that the plaster doesn't go off too quickly.

Without any pictures, it's only quess work, but you still need to hack off the blown plaster, and you could then apply bonding (or hardwall if you want) to bring it level, and the re plaster.

 

[derekphl]

The PVA I applied originally was diluted about 1:1, it wasn't pure; that was only a question about whether I should use undiluted PVA, which from your comment doesn't sound like a good idea in any case!

This is the wall in question, prepared yesterday (I cut a line as the below and left of the window was sound and skimmed without issue).

As hardwall seems to expect a dry medium porous substrate, I wasn't sure if stabiliser was necessary or even a good idea - maybe it would interfere with the adhesion to the brick?

 

[Doggit]

You can use Hardwall, or bonding as a backing to the milti finish, but having a wooden beam in the wall, I'd use bonding with a 1:3 pva first, and then pva again before you plaster. And no, you wouldn't use stabiliser in this situation.

 

[derekphl]

Splendid, thanks!

 

[derekphl]

In the end, I took a slightly different approach to the wood beam. I cut a piece of plasterboard to size and screwed it to the beam, then used hardwall (as I had already bought it) followed by multi-finish.

I was a little worried because the finish plaster seemed to harden a hell of a lot quicker than on the first wall I did despite the hardwall being moist still. But by the time I had the second coat on, it seemed to be picking up nicely.

I've only just done the last trowelling, but I'm pleased with the result.

 

[Doggit]

Well done Dereck, You've every right to be pleased with yourself, and yes, good idea on the plasterboard.

 

[derekphl]

Thanks Doggit!