Paint !
Just to stir things further I will also point out the largest purchaser of all of the F100 was American Airlines (75 frames, about one third of the total production) and for their livery they do not paint the aircraft AT ALL, as is well known (apart from a little red/white/blue detail line down the windowline of the aircraft). Everything else is plain unpainted alloy.
Which, if you know your aviation history from the 1940s/1950s, is how they all were .....