Mustangs had the top sides of their wings painted with a silver dope at production, which was worked over the surface to fill and smooth any rivets and any non-removable panel lines, in accordance with the laminar flow properties of the wing design. It was for aerodynamics, and the stuff dulled almost immediately once subjected to the sunlight and weather. It is well documented, and often overlooked in P-51 builds, as people get excited about re-creating shiny metal panels!