Urgent advice please - crazing on finish coat

[inwood]

Can anyone advise on the cause/solution for crazing over my finish coat. This is my first attempt at lime plastering a finish coat although I do have lots of plastering experience with gypsum.

Mix: 1:1 lime:silver sand (kiln dried block paving sand), no water added.
Background: Dry devil coat (completed about 3 months ago)
Preparation: Wetting of surface about 6 times to ensure a consistent colour across the wall, with no 'standing water'
Application: two tight coats perpendicular to each other, sponge floated second coat, third coat applied into this and then trowelled off in two directions. Lovely finish (crazing aside) - perhaps too overtrowelled if anything.

I applied the finish coat on Monday (now Wednesday), but am now finding a fine crazing in some areas. These are particularly prevalent around the edges of the walls but there is a patch about 20cm square offcentre from the middle of the wall.

So my questions are:
1. Is this crazing primarily caused by shrinkage? - i.e. should I make sure I have a more damp background for future walls.
2. The crazing is hardly noticeable to the touch and I've been pushing it in as it's been appearing - is it likely to get worse?
3. What aftercare do I need to apply and for how long? - I've given the wall a light spray morning and afternoon. I'm off on holiday from Friday for two weeks so don't won't to come back to a jigsaw finish across the whole wall.
4. If cracks do appear how best do I deal with them? - fill with lime putty?

Any advice gratefully received.

 

[BMcD]

It sounds like shrinkage cracks. I use a very fine sponge float (like a tile grouting float but without the rubber) for scouring and also scour the 3rd coat before troweling to compact the sand/lime together. Also you may have troweled the lime out of the finish, it needs very little water on the wall, generally I just wet the trowel. If they are very fine hairline cracks then a good few coats of limewash should heal them up.
You could always try a richer lime mix for fine work say 3:2 lime:sand I find this an excellent plaster for fine finished work

 

[inwood]

Thanks BMcD, it was from one of your previous posts that I'd read your finishing technique and which I used here. Gives a great finish, very smooth.

I think the spongfloat I'm using is too rough and soft - it's just like a standard sponge with lots of open texture. Because it's so soft I can't get any pressure into the plaster - it's more like moving the plaster around. I'll give the grouting float a try as you suggest - I have an old one without the rubber. I'll also give a richer mix a try.

Thought a photo (not great quality I'm afraid) might help to visualise the problem:

 

[Ian Bond]

I've used lime putty to fill very fine cracks (historic and new) in plaster, and it works well.

Some cracking seems unavoidable, despite best efforts such as good dampening down before starting, tending the plaster and mist spraying.

I've just cleaned up some very old plaster, and by the looks of it, that had fine cracks in it when it was done 170 odd years ago. Several coats of limewash sorted it out then, and should do now.

(Just removed wallpaper, pink emulsion, gypsum skim etc from all the walls and ceiling of a room, repaired the plaster and have started limewashing)

 

[Jon Maine]

Spray down a few times to make nice and damp, use just neat lime putty, you may find it useful to run it through a fine sieve or buy some super fine putty. Just float/ trowel/sponge this into the surface and refinish with a good fine trowel.
Or several coats of limewash should have the same effect.

Just keep a check on the soundness of the surface, tap it make sure its not gone hollow anywhere.

Good luck.

 

[BMcD]

By the look of the picture the crazing may have been caused by rapid suction , did you spay the very top of the wall or stop just short to avoid soaking finished work? or is not just the top 6"? But to be honest it doesn't look too bad and I don't imagine it to get worse, it should easily cover with lime wash. You may find it also pays to buy in ready mixed finish as the putty is well matured and the mix is consistent, with nothing more to do to it than whizz it up with a mixing paddle.

 

[Jon Maine]

Its caused by shrinkage, either too much suction i.e. not enough damping down, drying out to quickly especially by windows or draughty areas (shrinkage) or not enough aggregate in the mix ( again shrinkage) or possibly a poorly mixed top coat.

 

[Gervase]

Aye, shrinkage. It could be overtrowelling - if the mix is very wet, a steel trowel can lift a significant amount of lime to the surface where it will craze when it dries. Or it could be that the finish dried too quickly. It doesn't look too serious, though, and a coat of limewash would put it right.

 

[Ian Bond]

My first attempt resulting in crazing that was so bad the individual 'scales' were loose, and moved when lightly tapped with a finger. Scraped it off and tried again.

After much trial and error, I can now get reasonable results, and do not worry about slight surface crazing in small patches.

My 'pump up' garden sprayer was the best purchase I made, and beats trying to throw water on the wall with a brush. And it's great for keeping the plaster damp whilst it sets.

Perhaps you could get a neighbour in to water the plants, and the walls whilst you're away

 

[inwood]

Thanks to all for your words of reassurance. All appears well. There's no holloness to these areas and the crazing doesn't appear to have turned into cracks or got worse.

Although I did spray the whole wall fairly well it clearly could do with more. The ceilings in this room are plasterboarded :roll: and when I skimmed them I did get small amounts of gypsum on the devil coat of the wall where it meets the celiing. I suspect having that there didn't help as there was probably differential suction between lime and gypsum.

I'll make sure the next wall has a damper background to manage the suction better and get rid of the horrible gypsum :wink: