Unpaintable plastic

When OTFCC 2003 Sideswipe was retooled to remove the police sirens, a patch of white unpaintable plastic was revealed 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.

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