What is the purpose of the adhesion promoter when painting plastics?

The purpose of the adhesion promoter when painting plastics is primarily to augment the mechanical "tooth" provided by sanding. When plastic surfaces are sanded, they gain microscopic texture, which allows paint to grip better and adhere effectively. However, plastics often have smooth and non-porous surfaces that can still present challenges for paint adhesion. An adhesion promoter serves to improve the surface's ability to bond with the paint, creating a better mechanical connection and reducing the risk of peeling or flaking.

Enhancing gloss, sealing the surface, or acting as a base layer are not the primary functions of an adhesion promoter. A gloss enhancer would simply improve the shine of the paint, while sealing would protect the plastic rather than promote adhesion. Acting as a base layer typically involves a primer used for specific types of surfaces or conditions, which is different from the role of an adhesion promoter in facilitating paint adherence to plastics.