VALVOLINER
Super Freak
No they are not rubber, in fact i cant feel any rubber anywhere on him
I have'nt the Mark III BD, but the Mark IV pelvic piece is made of a flexible material (not rubber).
For this reason, The Mark IV has a great range of motion in the hip joints, although at first glance, it may looks the same hard plastic and paint than the rest of the armor.
This is exactly the same material for the red extra collar (Downey jr sculp)