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 Rendering with Sand cement lime mix, help needed 
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Joined: 19 Jun 2004, 16:55
Posts: 245
Location: Manchester + Dordogne (24800)
I have a few old exterier walls that I have scratch coated with a sharp sand/hydraulic lime/white cement mix 5:2:1 ratio, which a friend told me. The scratch coat has dried and cracked a little, which I expect is normal?

I applied this 2 weeks ago and now need to apply top coat and then rub up with a plastic float and finally a sponge which I think is how the french do this, but unsure.....I think I need to dampen down the areas first and then apply a weaker top coat, but not sure of the ratios of sand/hydraulic lime and white cement which I think needs to be a weaker mix?

I am fairly competant with rendering with just sand and cement in the UK, but not sure how to proceed with the top coat with sand/lime/cement as I do not want it all to crack up.....

Hopefully the experts out there can help me.....

Kind regards,

John


16 Apr 2012, 18:09
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Joined: 04 May 2008, 04:46
Posts: 10837
Location: Central Brittany
Not sure why you are using cement in the mix as there is no need. 3 or 4 to one sand (02) to lime for top coat.

Wet the bottom coat, then using a normal float do your top coat.

At the end of the day using a sponge float (a sponge on a board with a handle) go back over what you have done oldest first. Dont be bashful about how wet the sponge is.
In the morning check for cracks and sponge them out
later that day sponge out any other cracks and do that for a few days and you will be sweet.

see page 6 for how it goes on and here is an outside shot although we use 3 sand 1 lime and one sieved earth
This was my first ever go and thankfully I have improved my technique somewhat :lol:
http://i455.photobucket.com/albums/qq27 ... CF1425.jpg

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16 Apr 2012, 19:58
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Joined: 19 Jun 2004, 16:55
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Location: Manchester + Dordogne (24800)
Thanks for the info Bently....really helpfull
I think that with the white cement it would set quicker? Maybe this was a pointing mix, I think I was wrongly informed.

Is the ratio with sand to lime 4 to 1?

I am using st astier hydraulic lime in the green bags...


16 Apr 2012, 20:17
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Joined: 04 May 2008, 04:46
Posts: 10837
Location: Central Brittany
I think your cracking in the scratch coat comes from it drying out too quickly and adding cement is the likely cause of that quicker drying out.

The way lime mortar works is through calcification. It absorbs CO2 and basically reverts back to the stone it was made from.

Cement is different. (sorry that is a bit of a cop out but I have pasted some info for your perusal.)

Lime mortar on its own will give you a strong durable finish. Use 3 sand to one Hyadraulic lime (the stuff you mentioned) using 02 grade sand for the finish coat (a grittier sand 04 for the scratch coats).
Damp down the scratch coat well before applying the top coat to help it bond, hopefully it is well scratched.

http://www.buildingconservation.com/art ... cement.htm

This is a good article and supports the 3 to one mix.
http://www.minervaconservation.com/arti ... nders.html


(ps I am not a professional builder , just an enthusiastic amateur who has listened to and learnt from good builders and through trial and error. My renovation is in Brittany and the mixes and finishes bli end in with what's around )


Peoples opinion varies on this.

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16 Apr 2012, 23:19
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Joined: 19 Jun 2004, 16:55
Posts: 245
Location: Manchester + Dordogne (24800)
You were spot on Bently....3 to 1 sand/ hydraulic lime with no cement.
An hour or so after applying the second coat, it rubs up easily with a plastic float. I had various thicknesses from 10 to 25 mm to level it out, so I just rubbed it up slowly and come back an hour later when the mix on the wall was not "moving" and had taken a set. (started to harden)

Brilliant.....many thanks for the info....


17 Apr 2012, 19:07
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Joined: 04 May 2008, 04:46
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Location: Central Brittany
Buy one of those sponge floats as they are amazing. If you get any cracking or crazing after a day or two just sponge them out You can still sponge it up to few days later with plenty of water. Give it a few weeks to dry out properly.

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17 Apr 2012, 22:15
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Joined: 15 Jun 2008, 10:24
Posts: 489
Location: Loches
You could also add RENACAL to your lime/sand mix, so 1 lime, 1 renacal to either 3 or 4 sand (02) not (04) this will make your mix a bit more 'sticky' and easier to work with. Most builders merchants sell this but I have not seen it in the brico sheds.


20 Apr 2012, 06:40
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