Render - Waterproofer in both coats?

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FreeD

FreeD

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Working for a builder at the mo who said other guys hes worked alongside put waterproofer in scratch and top coat. I was told at college to put waterproofer in scatch, platiciser in top coat. He asked me whats wrong with putting it in both...i wasn't sure what to say, what is the reason?

Thanks
 
I have mostly put waterproofer in both coats unless secified not too, different arcitects specify different mixes it all depends on the job, Water proofer does exactly what it says it does creates a waterproof barrier so why not put it in the top coat i have done render jobs and they havent been painted it for years and i know when water proofer is not added to sand and cement it sucks up water like a sponge. I have worked with spreads who claim putting waterproofer in the top coat helps shrink the render and cracks it but if the scratch coat is left a week or so to cure the cracking is a little less,
 
Putting it in the top coat would also create a barrier between the top and scratch coat, so water from scatch coat has nowhere to go, unless you wait until scatch coat is completely dry?. Adding platiciser to top coat increases elastcity so helps prevent cracking.
 
As bodplasterer say if does depend on the job, i would never use a mortar plastiscer in render, over mixing or plastisceing only weekens it, render shouldnt be light and easy to spread save that for internal stuff, lime will make it workable and it will add elastcity and helps stop strinkage  it also helps retain moisture ,waterproofer does not make a compleat  waterproof barrier it will still allow the render to breath, all plasterers do things different ways you have do what you think is right at the end of the day for yourself,
 
The reason its put in the scratch and not the top is its meant to breath, water will soak into the top coat and then dry out, if its in the top water will still soak in but cant freely get out and if it freezes say goodbye to the render. Some times waterproofer is used in some types of render like ashlar, this is three coat work and decorative.
 
most buildiing renders do not have to breath especially over new blockwork and waterproofer will not stop it from breathing ,most modern exterior paints are not breathable lime renders are used for this purpose.
 
i meant the building needs to breath, what has new blockwork got to do with anything?
Exterior paint has only been around for ?, lime is also added to rendering in the top coat without waterproofer.
 
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