Which topcoat is best suited for smoothing?

The Baumit is also suitable for the base area but has a slightly smaller grain size. So it shouldn't be a big difference? Should you always apply two layers? I would first smooth the existing plaster a bit, for example with a concrete grinder or by hand with the rabott, then apply the bonding plaster and then smooth it a bit with the facade spatula, and the next day sand it with the eccentric sander. Or does it still have to be smoothed with a sponge somehow on the same day?

The material from Knauf really sticks well and can be applied thinner. It is more expensive but less complicated. A bonding plaster may need to be pretreated, for example with a mortar like SM. Due to the roughness of the substrates, it might work though. It's always difficult to judge from photos. Two layers usually result in a nicer texture with a float finish because you get an even application and the mortar sets evenly. It's all a matter of personal preference. I would work with two layers.

Ok thanks. Then I'll order it.

And what about smoothing? Should it be sanded the next day, or wiped over with the sponge after about 30 minutes?

The bonding plaster and the SM 700 should both be rubbed down. Ideally with a sponge float. A sponge also works, but it will make the surface more uneven. Both are mortars based on lime-cement, which means they harden relatively quickly, and due to the grain size, sanding will not achieve the desired effect. The timing of when to rub down depends on many factors. It is difficult to explain. It depends on the weather, the substrate, and the plaster thickness. It must no longer be soft; otherwise, the mortar will be rubbed away too much again.

However, this will not be a smooth plaster like you usually have indoors. Search for images of Filzputz. The visible grain can be somewhat reduced if rubbed down later or lightly scratched after hardening. You have to try it.

Ok thanks, I will give it a try. It can hardly get any worse...