Blown plaster options

[Mart]

We have begun stripping wallpaper and as suspected there is a rather large patch (approx 2m2) of blown plaster which is visibly flexing/crunching when pressure is applied. I wouldn't be surprised if there are other patches blown further up the stairs.

It's mostly intact with hairline cracks and a couple of old rawl plugs embedded in it, seems to be a fairly thin top layer which has separated but hard to say for sure, also all lime based by the looks of it.

This is on a large external wall and i'm not sure how to proceed, i suspect removing the blown patches will open a can of worms and replastering the entire wall would likely be prohibitively expensive unless that is my only option.

How would you proceed?

 

[a twig]

Urgh, we had yellow stuff like that in our kitchen, horrible.

It will be very unlikely that the brown stuff is lime based, but happy to be proved wrong!

Personally I would remove the brown stuff and break out the blown area and then repair as required. DIYable if you buy some premixed lime plaster and would save having to skim the whole wall.

 

[Cubist]

You could try to stabilise and consolidate it before doing anything else. In the past I've had some success with several coats of of a PVA and Water solution but I understand that there are other products that can be used. If nothing else it may help to reduce the amount of plaster that comes down as you remove the unsalvageable stuff. For a few quid and a little elbow grease it may be worth a try.

 

[Mart]

Urgh, we had yellow stuff like that in our kitchen, horrible.

It will be very unlikely that the brown stuff is lime based, but happy to be proved wrong!

Personally I would remove the brown stuff and break out the blown area and then repair as required. DIYable if you buy some premixed lime plaster and would save having to skim the whole wall.

Yes what is the yellow stuff?!

when you say brown stuff what do you mean? If you mean the small patches that is leftover backing from the wallpaper, the stripping wasn't very thorough so far.

It looks like behind the yellow layer it is all grey plaster with hairs in it which is why i was expecting it to be lime based.

If it's gypsum i'm less concerned about preserving it!

 

[Cubist]

Have you tried the 'fizz' test. A little vinegar painted on usually reacts with lime to produce a gentle fizz. You can't always rely on fibres in the render/base coat plaster as it was also sometimes used with gypsum products.

 

[malcolm]

I'm going through the same sort of thing. It's always best to limit repairs as far as possible as otherwise it turns into a whole lot of work.

It's common for lime walls to have been skimmed with a very thin layer of gypsum plaster. Over here the modern gypsum hasn't bonded too well to the plaster underneath and your cracks look familiar.

Where the gypsum is loose it'll need to come off. Just use a filler to replace it. Apply and form to shape with a straight edge. You'll need 2 or 3 applications and then a drywall sander is handy for finishing things off.

Steam wallpaper strippers tend to lift the gypsum skim coat from lime plastered walls. Something like Zinsser DIF wallpaper stripper instead of the steamer would reduce damage to the gypsum.

 

[Mart]

No discernible 'fizz' so perhaps its not lime or at least skimmed in something else.

Seems sensible course of action will be to strip the rest of the hallway to identify any other blown patches then remove and fill.

Would it be better to fill with a lime based plaster or any finishing plaster acceptable?

On closer inspection there's some suspicious bulges around the archway which is above the worse of the blowout

 

[Cubist]

Would it be better to fill with a lime based plaster or any finishing plaster acceptable?

If, you have a gypsum finish coat over a lime render coat then.... you're on the horns of a dilemma given that neither will bond all that well to the other so... damned either way.

In your shoes I think I would, after making good any weaknesses in the render coat, use a gypsum finishing coat - if that's what the current finish is - to get the best possible joins at the surface.

 

[Stevers]

When patching old plaster I've had huge success with dilute PVA to stabilise whatever is under the skim, and finishing with Thistle One Coat. The One Coat is more open textured than those shiny hard pink finishing plasters, and in my experience has much better adhesion. It's not as easy to get a mirror finish, so pros aren't fond of it, but that isn't what we should be seeking in an older building. It's only got to be good enough, and my earlier repairs have held up for 26 years now on a Victorian stone house from 1879 where I've patched lime over what looks more like dried mud than anything else.

 

[paulc]

A few patches of blown plaster, I have stabilised by injecting a thinned down grab adhesive in to the voids - The water based ones are best for this.
Skimming, I am currently leaning towards the Knauf Fill & Finish premixed compound. Once dry, it can be sanded, which is dusty, messy work. Alternatively, use a wet sponge float to do the sanding - The water softens the top layer, and the movement of the sponge takes out the humps and fills the hollows. With not a lot of practice, you can get a pretty decent finish with almost invisible feathering in.

 

[Stevers]

I should add that although One Coat can be applied to a depth of up to 1.5", I wouldn't dream of doing that, and always apply a render coat (or two) to any deep holes or bare masonry. Before I knew better I used 1:1:6 cement:lime:aggregate, and in the last 10 years 1:3 NHL:aggregate. On the Mendips at least the aggregate is generally 50/50 sand and stone dust. The 1:1:6 renders have performed well, but I much prefer the NHL mixes for workability.

One of my pet hates is the micro cracking that you get with the strong cement:sand renders. I was told to use these when making good rooms in our basement to meet the conditions of a probably worthless damp proof guarantee, and I won't be doing that again in the one room left to do.