Sabre wing question

The center wing structure of the Sabres was not likely paint, but a coating that made it look darker than surrounding panels. I usually paint my models with Alclad "dark aluminum" but add a bit more black to it to darken it more. This is a Hasegawa 1/72 F-86D Sabre Dog with Alclad finish.

The idea of a coating is interesting, I did not think of that. Maybe that's the reason it is less visable on highly pollished Sabres.

Maybe this centre section was prone to corosion more than the other areas due to a different alloy used?

I remember reading recently that this portion of the wing skin was made from a different metal. But I do not recall which one off the top of my head, nor  where I read that tidbit of information... Sorry I can't help more right now.