What make is that model? Maly Modelarz or Fly Model? I sure would love to help you with the translation from Polish if you need any. Those bulkheads and so on are usually reinforced with thick, consistent cardboard - ideally it should be about 1mm thick. If you sand the edges of such bulkhead or spar a little with sanding paper (about 120 is best), then you'll get some extra glueing surface. Where two segments join on a double bulkhead, you should hold the two bulkheads together and sand them together to ensure they have exactly the same shape. Then you put one bulkhead in one segment, second bulkhead in second segment, and when you join them you get a nice, smooth hull.
Good luck with your build and have a nice day