Using gypsum with lime plaster wall

[Riyock]

I need some advice please. Novice DIYer who’s been doing a lot of research on how to fix my walls myself…

I stripped the wallpaper off the wall here expecting to be able to paint it. Turns out the plaster is in a poor state. Some of it is hollow. So I started to remove parts of it but stopped as I realised that I’d end up taking most of the wall off.

Since I don’t have the budget to get it skimmed, instead I thought I’d take a chance with what’s left, patch the holes and cracks with bonding, scrim and then use roll on plaster (Dalapro Nova) with a layer of render mesh for reinforcement. I know roll on is not how we traditionally do it but apparently the results are decent if you aren’t a plasterer and can’t afford to call the pros in.

However, I then found out about lime plaster. Popped some of this in vinegar and it does seem to fizz… I was surprised as this is a 1960 house so not that old, plus it appears to have gypsum (the pink stuff) on top of the plaster. I think what’s happened is I’ve got blockwork with some kind of lime render on top that at some point someone has skimmed over with gypsum when they tried to modernise it.

Apparently gypsum and lime do not mix because the gypsum doesn’t breathe and it could trap moisture leading to damp. Plus it’s not flexible which means eventually the lime will crack around the border with the gypsum.

So I think I need to change my plan. I’d patch the big holes with a lime mortar, and a lime filler for the other cracks. I’d then line the walls with lining paper and then paint that.

My question is if it’s worth using lime if there’s already gypsum on top of the lime. Seems like it’s already lost its breathability so is there any benefit here? From what I can see using binding and easy fill is a lot simpler than lime mortar and filler.

Also I’m assuming I am dealing with lime here based on the vinegar experiment, although I do wonder if actually it’s something else that’s just having a bit of a reaction? Keen to hear opinions on that. I’ve posted a video here:

I should mention that this is an upstairs bedroom with one external wall that has a patch missing like in the photo. However, the property doesn’t have any damp problems and the other rooms also have this gypsum on lime, painted wallpaper situation.

Any and all help here greatly appreciated!

 

[StephenStephen]

Personally I use lime putty plaster for this kind of job, for the big patch, and for the cracks, not because of concerns about breathability, but because it goes off much slower so I'm less scared of using it as an amateur than I am of using gypsum.

It is harder to get hold of and more expensive to buy.
Have a look at the Mike Wye videos - they're either on their website or on YouTube.

 

[Mrxman]

That looks like lime to me. I've done a similar project although a lot fewer cracks. Our house is also lime and I filled holes with regular modern plaster mostly out of ignorance. One thing I'll say is that some of the cracks slowly reappeared after 2 years probably because of slight movement.
It's obviously difficult to make recommendations based on photos and I am by no means an expert. What I can share is how I dealt with cracks in the end. I bought some toupret fill flex and then put some plastering mesh tape on top. Then used toupret ready mix smoothing filler on top of that. The trick with smoothing filler is use a large flexible filler knife. You probably need 2-3 layers. It takes forever but sands well which is the key to a good finish diy stylee.

 

[JohnD]

It's lime plaster, with the remains of distemper on it. But why is it cracked? Looks like the wall is moving, or has moved.

Have a look at the outside.

Is it built of stone?

Are the doorframes square and vertical?

 

[JohnD]

BTW I have sometimes bodged live plaster by injecting PVA adhesive behind it and pressing it into place. Not exactly a professional repair.

 

[opps]

Not my area of expertise, but those look like cinder blocks. They were made out of "slag" from coal mining.

Until recently, I lived in a 50's property. The walls were cinder block but they were internally skimmed with an incredibly hard cement finish (presumably because the blocks are so weak).

Our walls didn't have lime plaster though.

Are your external walls rendered?

Again, not my area of expertise, but cinder blocks were used because they were cheap. I wonder if perhaps they used lime in the cement finish skim to make it easier to work with.

Can you significantly scratch the finish if you run a screwdriver over the skim coat. I would expect lime plaster to scratch with ease but not cement.

If not, and you want to redec as cheaply as possible. As per @JohnD 's recommendation, brush some PVA into the cracks. It often helps to lock cracks. Then buy some Toupret Flex and Fill. It is like a caulk with fibres in it. You let it shrink back over a day or two (depending on the size of the crack).

As a decorator, I would then use Toupret TX110 filler where required and line the walls. It is cement based so deep fills harden quicker. It is only about 25%-30% more expensive than the regular Toupret filler. And, yes, as you suggested, lining paper over the whole lot. Possibly 1400 grade. 1000 if you plan to cross line.