Unpaintable plastic

Just what its name implies, unpaintable plastic is plastic to which paint will not adhere, due to the chemical compositions of said plastic. This plastic is sometimes used on Transformers toys for structural reasons, due to its durability, flexibility, or resiliency.

Toys known to use unpaintable plastic include the Robots in Disguise Prowl mold and Energon High Wire.

From Teletraan I: The Transformers Wiki, a Wikia wiki.