How was Vader's helmet modified from ANH to ESB to ROTJ

[Imperial Weapon]

One thing I was wondering about the ANH chest box is if it was sprayed silver first then the black was chipped away or if was just simply exposing white gel coat, now I understand that this is a helmet thread but its just one detail on the ANH Vader thats been nagging me.

 

[joevader]

Here is Mike Warren's ROTJ, next to a GH ROTJ.