Technical point: The Chassis is the frame upon which all the components are mounted. It is not usually visible to the user. It often serves the vital purpose of being common ground. Coating it with any paint or cosmetic coating would interefere with this function.
I think you are referring to the Case. The case covers all the components and chassis, serving as both a protective shield from electrical components, dust cover and cosmetic enhancement.
p.s. Modern radios will typically have a chassis made of stamped steel or stamped, cast aluminum or cast zinc. Boat anchors may have a chassis made of steel, aluminum and protected by an anti corrosion layer such as cadmium, zinc or tin. It is important when cleaning or refurbishing not to damage or remove this protective coating.