Thank you for your positive replies. I moved a bit forwards with this build again. One of the shortcomings of the kit in my view is the thick windows framing. There is no such thing in the real bus. It is more like a wall with windows cut in. In order to get a look like that, I covered the windows with a 0.3 mm sheet which I glued to the frames and then I cut off the windows.
Now with a surfacer applied it looks like this.
It needs a bit more work on the shape and surface but I think that this modification helps the interior look better.