Fresh Lime Putty Cracks - beyond help?

[sixeighth]

Help avoid another diy disaster - please!

I'm belatedly seeking help with applying lime putty which I've been using for patching on old lime-rendered walls (tenement flat in Scotland) but have not managed to avoid cracking in the finish.

The latest attempt is a fire opening which I'm getting ready to have a stove fitted. A first coat, with hair, went on and dried fine but the finish coat is showing cracks as in the pic. I'm using pre-mixed bagged 'rendering stuff'.

CrackedPutty

• sixeighth
• 15 Sep 2011
• 1

Cracked Putty

I thought I had it sussed when I realised that you're supposed to keep the putty damp as it dries but some cracks appeared after a couple of days. The wall didn't get it's usual soaking yesterday and lots more cracks are appearing - five days since the finish coat went on.

I did the sides of the opening with hydrolic lime render a couple of days ago (stove was due to be fitted this week - now postponed) and they look fine at the moment but is there any hope for the back wall or will I have to knock it all off?

The stove being fitted is convector sides and back, radiant at the front.

 

[joe-90]

Did you soak the walls before you started?

 

[sixeighth]

Did you soak the walls before you started?

Yes, sprayed regularly over a couple of hours before starting.

 

[Possel]

I thought I had it sussed when I realised that you're supposed to keep the putty damp as it dries but some cracks appeared after a couple of days. The wall didn't get it's usual soaking yesterday and lots more cracks are appearing - five days since the finish coat went on.

Do you mean you've been spraying it repeatedly AFTER application of the finish coat? This should not be necessary and could be the cause of your problem. You have to prevent it from drying out too rapidly, which is a different thing, by (e.g.) covering with damp hessian if it's really hot. NB The wall has to be dampened before application, not "soaked". You are reducing the suction of the dry wall so that the mortar does not dry too quickly (and it will then fall off).

If it has been dampened regularly, it might still be workable - try tightening the surface with your trowel.

If it has cracked irrepairably and is unworkable, then wait and see if the plaster stays attached. If so, all you have to do is to apply a skim of some lime finish coat over it. OK, if it's loose then it has to come off.

 

[peaps]

What mix was the hydrolic lime render?
How thick was the float coat?
How warm was the room?


Lime has a tendency to crack after application and needs time to cure. Rapid drying is the problem but this can be caused by many reasons.

Lime that is applied too thick will crack.
Lime that is applied to high suction backgrounds.
Athmospheric conditions.

You can still rub it up weeks after you apply, depending on the mix however.
I would give the wall a real big soaking, you don't want the lime render to be sucking in any water, then give it a good rub up with a plastic float and keep it damp.

If you have more work to do I would suggest you clean (hoover) the area and soak the wall and I mean give it a good soak! If you have a deep patch to do build it up with scratch coats about 8/10mm thick. If it takes 3 scratch coat then it needs three. Lime render is not a quick fix so to speak. It takes time, lots of time.

Hydrolic lime has no impurities in it so it needs to have cement added to it hence the 1.1.6 lime mix

 

[Possel]

Hydrolic lime has no impurities in it so it needs to have cement added to it hence the 1.1.6 lime mix

Which surely makes it impervious and defeats the object? Why add anything to hydraulic lime in this situation? NHL sets perfectly well without cement.

Edited to add: The reason why hydraulic lime sets without air is precisely because it DOES already have impurities in it!

 

[sixeighth]

Many thanks for the replies and sorry for the delay in getting back.

Possel - the scratch coat was well dampened. Water wasn't running off but it may have been dampened too much. I did spray it with water regularly after the finish (fairly fine mist from a 1ltr plant spray).

I can rub it up a bit so the cracks are filled but it doesn't last long before they open and the grit it creating a few 'gouges' - I'm not too worried about those though or the cracks as long as it's sound (have to take a chance as the stove is being fitted next week). I've used a tub of finishing stuff before but had the same problem with that I'm afraid. Is lime finish stuff different?

peaps I don't know what mix the hydrolic stuff is as it came ready-made (dry) but it looks fine at the moment, it's the putty that's cracking.

...If you have a deep patch to do build it up with scratch coats about 8/10mm thick. If it takes 3 scratch coat then it needs three. Lime render is not a quick fix so to speak. It takes time, lots of time.

The bottom of the wall where the putty is at it's thinnest does not have any cracks so I may have it on too thick further up given what you say. Looking like I should have taken more time and given it an extra coat to build it up properly.[/b]

 

[Possel]

I've used a tub of finishing stuff before but had the same problem with that I'm afraid. Is lime finish stuff different?

What are you using as "finishing stuff"? For my house I used 3:2 fine sand: lime putty, purchased ready mixed from a well known supplier in Devon. This would give an acceptable finish but we added another coat of their "Regency plaster" which is (I think) 1:1 lime putty: fine sand and marble flour. This gives a really smooth surface if applied properly.

 

[sixeighth]

This gives a really smooth surface if applied properly.

I might be fine if I'd applied existing properly.

I checked the tub and it's called 'lime putty setting stuff' not finishing stuff. The supplier is a local supplier of lime mortar who have been very good with their advice so far (I don't ask enough questions though) and I think their reputation is very good. I've since found out that they run short courses - which would have benefited me at the start of this project.

I'd settle for a 'rustic' finish if it stays on the wall.

 

[peaps]

Which surely makes it impervious and defeats the object? Why add anything to hydraulic lime in this situation? NHL sets perfectly well without cement.

Cement has been mixed with hydraulic lime for generations. Adding cement would strengthen the render used inside the fireplace since hydraulic based plasters are weak. We have no clue what type of hydraulic lime was used so we are both guessing....

Who said anything about adding cement to lime so it sets?

Edited to add: The reason why hydraulic lime sets without air is precisely because it DOES already have impurities in it!

"If the limestone contains particles of clay, after burning at 950-1200°C and slaking, the lime produced sets by reaction with water. Limestone containing the lowest proportion of clay (less than 12 per cent) results in a feebly hydraulic lime with properties close to non-hydraulic lime, which is relatively weak, permeable and porous. Higher proportions result in successively stronger and less permeable lime mortars. Because they react with water, hydraulic limes are usually supplied to site as dry powder. However, they can also be made by dry-slaking on site and may be knocked up with water and banked on site for a few days. "

Pre victoria lime had more than a touch of clay in it and is the reason we can't quite get that perfect lime putty mix these days. I send a sample of to be analysed and the mix is never the same....

http://www.buildingconservation.com/articles/limebasic/limebasic.htm[/b]