Years ago I got a 1/24th scale MPC (airfix) Stuka kit. (1980 btw) Built it painted it up. The decals were NOT decal material. They were like mylar. They wouldn't stick, they just curled off. They were so thick that you could hold and handle them.
As this was 1980 and decals were not available especially in this size, I had only one option. I used the decals to make my own masks. I used tracing paper, and Scotch double stick tape on the under side. Masked it, and airbrushed the markings with my original single action Badger airbrush. For the time and my experience, it turned out quite well. I still have that model in pieces in a box, I should pull it out and rebuild it. (Sometime)