I’m not sure but, as a hero with the face entirely covered by a mask, Nanairo Kamen (Seven-colored Mask) might be the first one.
It’s a superhero featured in a monochrome TV series “Nanairo Kamen” (broadcast from 1959 to 1960).
Along with “Gekko Kamen” (Moonlight Mask; broadcast from 1958 to 59), he’s one of the heroes in the early period of such superhero TV series of Japan.
These two were produced by the same author, Kohan Kawauchi (1920-2008).
I don’t know much about this hero because it was broadcast before I was born.
It’s likely the mask material is unknown.
So the head of Magma Taishi may be the first one made of FRP.
It’s said Magma Taishi was supposed to be played by an actor exposing his gold-powdered face in the planning stage.
Magama Taishi with the actor’s face exposed can be actually seen in the pilot film.
It seems that, as the gold powder wet with sweat easily got off and so on, it was decided to use the FRP mask.
That shows the producers had a continuous process of trial and error in those days.
At any rate, you will be aware of the extraordinary uniqueness the appearance of Ultraman has among these superheroes.
I love the flavor of each one of them, though!!!