Thanks for the great answers, it is already somewhat clearer what has to be done. Although not quite everything - sorry I'm new to almost all of this.

So, since the TO build up should already be flat, I will wet sand with water and soap and wipe the excess away. And I'll use 1500 grit paper.
If I then build up TO layers and go to finer grits until i reach the desired gloss, do I even need to polish afterwards? If yes, how would I tackle that, after two weeks of wait?

Am I aiming for very thin TO coats now, or for medium/thick application?