I downloaded a decal from cosmos, copied the opacity map to the mask inside the decal and applied the material to the decal. I can see the square of the decal affecting the object it is being projected onto.
Edge of the vray decal gizmo seems to project a hard line on the surface (shown by arrow)
It’s almost like the opacity isn’t being taken into account properly and it’s reflecting in the paint of the tail fin.
This is the reflection filter channel with the decal shrunk right down.
So. It turned out it was the normal map for the geometry. It wasn’t setup correctly on the purchased model. All working as expected now 
