Render advice please

[Philthy]

What is the correct mix to render concrete walls that are below ground level, where one side has been excavated and the other side is earth. Concrete is about 4' high. Rough, uneven surface that I want to make flat to take brackets and fixings.
Cold damp will probably be a factor in winter. Humidity in summer. No winter heating source.

Do I need rendering sand (what grade is that)? Or concrete? Roughly what consistancy? Applying it won't be a problem; I've plastered walls before. It's the technical aspect of the mix that I'm unsure about.

[Scarlet]

I'm a complete numpty where diy is concerned but my brother is a builder ...not that I remembered exactly what this post said. But he said if it was inside and you needed to keep water out you would tank it first with a tanking mix which will stop the water coming through. If it's outside you can use SBR or a waterproofer and hardener in your render mix which will be 3parts sand to one cement.

[bearded bloke]

Yep, as Scarlet said it needs "tanking" in an ideal world, what you could do is apply 2/3 coats of Synthapruf #5 (or similar) then before the third coating dries throw "sharp" sand at it, let it dry fully, brush off any surplus sand then use a standard render mix.
Do however remember that by drilling a hole through the tanking you have breached the vertical damp course, so put some synthapruf into the hole BEFORE the rawlplug.