Lime Render Repair Cracks

[Flyfisher]

Any suggestions about how to avoid this sort of thing?

I used a 2:1 soft sand : lime putty mix to repair some holes and missing bits of render prior to painting. I misted the area before application and filled and smoothed a few cracks after a couple of hours before misting it again in the evening, but this morning it looked like this.

Some areas are about one inch thick so I'm wondering if I should have applied more than one layer?

It's not a big issue in this case because I can easily fill the cracks before painting so the end result should be ok, but I've got a few larger wall areas I was going tackle and am wondering how best to prevent cracking occuring in general.

 

[robgil]

2;1 is a tad hot is it not?
I probably would have used NHL and sharp sand myself. Did you put any hair into it?

 

[skier-hughes]

1" is a bit thick, misting the substrate is no good, needs soaking a few times.

 

[paulb33]

Has the adjacent timber sucked up moisture?

 

[Zebra]

I've not tried making a plaster/render out of just soft sand and putty, without adding any sharp sand, or using plastering sand, which has larger granules in it. It seems it'd be a bit lacking in body, so to speak, larger granules to lock it together.
You might use something like that - silver sand and putty - for a thin top coat, but in that case it wouldn't be that thick.
Looking at my lime plastering course notes: "the size of grain of largest aggregate needs to be third of thickness of coat" and "aggregate mustn’t be too soft & rounded, must be sharp to achieve locking strength" and "thickness of coats normally 9mm scratch coat, 9mm float coat, 3mm top coat."
You don't have to use hair unless it's going on laths, so I understand, although there are probably exceptions.
I have cheated a bit in places, and only done one 10mm coat with a sponged finish which is rather rough. What's the finish like that you're trying to match?
If you've got an inch thickness, that's 25mm (sorry - can't do imperial!) so probably an underneath layer of something a bit meatier would be the way to go - it doesn't matter if the bottom layer cracks. Then you could use the finer stuff on top, but again, probably not soft sand, at least, the soft sand we get round here is rather clayey and sticky. I understand that kiln-dried sand is used for a top coat, if you're looking for a smooth finish.

 

[Flyfisher]

Hmm. Perhaps I should consider a proper lime plastering course before doing tackling anything more substantial.

The underlying substrate is mainly daub so I probably didn't soak it enough to prevent too rapid drying out. I suspected I was being a bit optimistic given the thickness but was too impatient to try multiple coats and knew I could fill any cracks later. The existing render is fairly rough, so I don't need a particularly smooth finish and I've been using a 2-inch paintbrush (sorry Zebra, 50mm :wink: ) to smooth the edges anyway, which gives a lightly textured finish, and also makes it fairly easy to fill the cracks with a sort of 'lime slurry', so it's more of a 'filler' job prior to painting than proper rendering/plastering.

To put the job in a bit more context, this photo shows the location of the repairs, so they won't be getting much close scrutiny and the paint should cover a few sins.

Meanwhile, and rather more excitingly, we're about to have the front of the house professionally re-rendered so I'll be hoping to learn how to do things properly in future.

 

[Zebra]

Well, perhaps try some plastering sand, 3:1, if you're not after too smooth a finish. My sister and I plastered my whole bedroom with it, sponged off to give a rough texture. You might get the odd big stone which is a pain, but apart from that it's easy to use.

 

[plasticpigeon]

I think 1" is too much, I've never managed to lime plaster 1" without cracking. I also think soft sand is not so good. I would have another go in 2 layers, sharp sand for the bottom layer, and plastering sand for the top layer. I've found 3:1 works fine and it is nice and workable, but skim or setting coats are much more lime rich. Something like 2:1 or even 1:1 with silver sand, but I don't think that is appropriate for going between timber posts. That is my 2 pence worth. A sponge float is a godsend, if you haven't already got one, it will be the best £5 you've ever spent. Also I have found that compared to masonry, wood absorbs very little water, and I don't think it is good to wet wood before plastering, the water just seems to sit on the wood eventually making it swell and exacerbating cracks as it dries and shrinks. I don't think you should go on a course, but reading this book will be very helpful.

https://archive.org/details/plasteringplaind00mill