Difference between PVA or Lime in mix, and general questions.

[frank999]

Hi I had an external Thermalite block (the 2.8N/mm2) wall rendered a few years ago, PVA was used in the mix - its all still up and looking good.

I have just rendered an external Celcon block (3.6N/mm2) wall myself, I was advised to use Lime in the mix.

What is the difference, does the PVA simply ****** the suction ? where as the lime helps with the flexibility, to limit cracking ?

On a side issue, I painted a slurry directly onto the Celcon wall, with Bond-It SBR in the slurry - I understand this is used to ****** suction and help with priming.

I am applying the top coat a week after the scratch base coat - I assume simply wetting the wall and applying direct - it will key.

I am having trouble scratching - I need to leave the base coat long enough to sufficiently harden before I can feather edge it (without dragging off the render) but by this stage it can prove harder to scratch it - whats the best technique for the base coat - I see some dont feather edge - and simply scratch the freshly applied render.

 

[footprints]

I'm no Bricky but as no one else has replied, my take would be that PVA helps adhesion and waterproofs the mix to some extent, lime added to a bricklaying cement mortar gives it a "fatty" feel making it easier to work with and more flexible.
However I would think adding it to a render will not be much use unless it is a proper traditional lime mix that is breathable applied to something like a traditional building without a DPC.

 

[frank999]

Thanks foot print, I was told not to use PVA where rendering a block wall (which is quite weak) and make a weak mix of 1:1:6 (Cement:lime:sand).

I was advised to use SBR as a slurry for the primer (which I'm told also acts as a water proofer -ie it will stop moisture travelling through the wall), and advised to use Lime to help with workability (and allow moisture through the wall where needed - although thats not going to happen now SBR has been used)

I'd still like to know if putting the second coat onto the base scratch coat - will be fine by just wetting the wall, ie no additional primer needed.

 

[frank999]

I have found this document useful, even though it is for Mortar:
http://www.ibstock.com/pdfs/technical-support/TIS-23-Brickwork-Mortars.pdf

I was advised to soak the (now dry) first scratch coated wall, before applying the second coat - no SBR - just straight on, its cement.

Job done, looks a goodun ... using the 'foam float' to roughen the surface took a little practice, and good timing, not too early or not too late in the drying process.