Why does the concrete get "skin" that cracks...

[anders07]

Poured more on the patio, about 5 sqm, and mixed myself with a tumbler according to instructions. Finja Grovbetong in 25kg bags directly from the hardware store.
The surface is wood-finished.

There is a thin skin on the concrete, about 1 mm thick, that is peeling off now after two days. I started watering about 4-5 hours after pouring and covered it with a tarp.

I have poured a whole lot before but have never gotten a similar film...

 

[hul]

do not wait 4-5 hours to water. you have surely treated the slab on a warm day, so you have brought the cement to the surface, which has hardened quickly and cracks

Water at the same time as you pour, stop only when you are treating the surface, immediately after start watering again and cover the surface.
alternatively, use a membrane curing agent

 

[anders07]

Well, it was warm... I went over with a plank and scraped off the surface, I guess that's what brought up the cement...

The surface underneath seems fine though, so I'll just have to polish it a bit with stone.

 

[hul]

Yes, it was warm... I went over it with a plank and scraped off the surface, I guess that's what pulled up the cement then...

However, the surface underneath seems completely fine, so I'll just sand it down a bit with stone.

No idea how big the surface is but rent a grinder and sand the surface, and you'll get a great and nice-looking surface.
More durable than a normal surface too.