Please do not use too much water. It is best to use at least 2 A4 sheets to have enough material available. I previously mixed the gray according to references from aircraft carrier kits. You can often find these painting instructions on the Internet from kit reviews (you do not necessarily have to buy them). After painting, let the cardboard dry a little and press it between books so that it becomes smooth and plain again.

3.) Next I drew and cut out the basic shape. I sketched the layout on the computer to find out what the layout should look like before I cut it out of my painted cardboard. Then I printed it out on simple printer paper, cut it out and transferred the shape to my previously painted cardboard.
Since the whole thing is only supposed to serve as a photo background, I only built half the shape of the superstructure, just like the backdrops in the theater or in filmproductions. The portholes were punched with a punch. I used paper glue and rubber glue for gluing. After assembly, I added small doors, trusses and borders. In the end I painted the white paper edges again with paint and then aged everything with my airbrush.

Hope this will inspire you to make your own backdrop, give it a try!