Step 6: Final layers of concrete
I have read two conflicting statements about adding layers to concrete: that you should do it before the layers underneath have fully set, and that you should do it after. I'm not sure which is true, but I can say that I did it before it was fully set, but ater it had hardened considerably.
The next layer should be smooth concrete. After a fair bit of research, I came up with this recipe: 1 part portland cement to 1 part silica flour (by weight). Mix them thoroughly dry (don't forget to wear a mask!), and then add water as before, just so that the cement is workable.
Again apply a layer of this cement. You can sand it down to get a very smooth finish, as you can see in the photos.
You'll need to cover this/keep it moist as well, so that it properly cures without cracking.