Unpaintable plastic

Due to Prowl's mold layout, changing the outer shell of car while leaving the limbs white for Sideswipe left a white part on the roof that had to be covered by a sticker. All of Sideswipe's white plastic (and the corresponding parts on other versions of the mold) cannot hold paint.

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.

The two most common plastics that are unpaintable and often used in Transformers are Polyoxymethylene(POM) and Polyamide(PA). The vast majority of Transformers toys have at least one mold that is unpaintable.

Examples

• OTFCC 2003 Sideswipe -- See photo and caption to right.
• The Ultimate Battle Optimus Prime -- Much of his red plastic, especially the upper arms, is made of unpaintable plastic. This is why the Autobot symbol tampograph was moved to his forearm.
• It was believed for a long while that the plastic on Beast Wars Dinobot that included the robot legs was unpaintable, since every version of that mold did not have paint anywhere on parts that shared that plastic color. However, both the American and Japanese versions of Tenth Anniversary Dinobot do paint the legs, so it is possible that either fan suspicions were wrong, that the mold has been modified to be able to use painted plastic there, or that those parts can be paintable plastic without sacrificing structrual integrity but weren't previously. Aaron Archer was asked about this prior to the Tenth Anniversary toy's release, and he assumed Dinobot's legs were unpaintable plastic because they had never been painted.
  
  
  From Teletraan I: The Transformers Wiki, a Wikia wiki.