Render mix

[cwr]

Hi folks. 

What are people's opinions on sand cement render mix? 4:1 then 5:1 seem to be commonly recommended on the internet, but I'm unclear on the best approach to the addition of lime/plasticiser/waterproofer.

I've concrete block garden walls to do, and at some stage single skin garage walls, also concrete block. All will ultimately be painted white.

Thanks