Max

Hi @SetLeit,
Honestly, I'm unsure how to guide you exactly here as we don't internally use Blender (We use 3ds Max) and supplied the Blender kit as is based on community request and I've had no complaints about the kit to date brought up to me directly. You can see we also don't mention the Blender file in our paint kit manual.
Maybe there's a way you can select which material/texture your 3D painted result outputs to? By your screenshot, it looks like it's outputting to all materials assigned to the fuselage object, when it should only output to the material on the relevant section you're painting your decal onto in the 3D viewport.
Alternatively, you can also try separating the mesh by material by selecting the fuselage part by the material selection and then detaching it into its own object as shown below.



Regardless, I'm happy to guide you to the best of my limited Blender knowledge and help you troubleshoot this.

Hi @Phog,

Yes, we use relaxed UVs on the L1011, we found this method to be more optimal for specific sections of this aircraft. It didn't make much of a difference for us, since our in-house liveries are painted using 3D decals which is the best standard for quality, but I understand the traditional Photoshop painting method would be quite tricky with such UV layouts.

There is an .fbx model included for you to project your graphical elements onto the relaxed UVs using the likes of Substance Painter. This is the best and quickest solution.

Cheers.