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

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.

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.

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.

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.