Well, I'm doing the screeding by laying "rails", then filling in the
bays between. Mix is 1:3 cement: sharp sand, mixed with 1:3 SBR:water
(half-strength SBR mix) to semi-dry (stays clumped when you squeeze it
together).
Bonded with 1:1 water:SBR with cement to a creamy consistency.
Phew, laying screed is not easy !
I was doing a strip along one wall to get me started. 2 metres long by
200mm wide along the wall. I had a guide to level to (a sort of frame
made of 25mm square steel tube). When doing the body of the room I'll
fill inside the frame to do the "rails", but by the wall I was filling
alongside the guide.
I filled to above level, compacted it, then sliced if off level using
a plasterers trowel. Trouble was, when slicing it off, it left behind
a rough surface with some pitting, due to the courseness of the sand.
I had to go over the area trying to fill some of the pitted areas.
Now, all this will be tiled so the pitting that remained did not
matter, but I was getting very frustrated.
Maybe I actually had it too dry, since it would not polish off like
concrete or mortar would.
After a while I hit upon a technique of slicing the surface whilst
sweeping the trowel in circles, which seemed to be good at filling in
the surface, helped by a little water from a sprayer.
Result, it is pretty flat though, find for tiling.
But obviously doing a larger area you have to smooth as you go.

So, what is the technique when scraping level a semi-dry sharp sand
mix, that creates a smooth surface as you go ?

I think this may be the hardest thing I have done so far (except
trying plastering !). Bricklaying is easy !
Cheers,
Simon.