Enclosures for electronic devices can have sections made of various different materials, such as metals, plastics, glass and ceramics. Each type of material has different functional attributes. Metals, for example, can have high tensile strength, can be electrically conductive and can provide a durable cosmetically appealing surface. Some metals, such as aluminum and aluminum alloys, can be anodized so as to form a hard anodic oxide coating that protects the underlying metal and can be readily colored to various shades. Non-metals, such as plastics, are generally radio frequency transparent, and therefore can be positioned over radio frequency transmitting antennas housed within the electronic device without interrupting radio frequency transmission, or can be used to electrically isolate various distinct metal parts of an aluminum enclosure to enable them to act as antennae.
One of the challenges with manufacturing of electronic device enclosures relates to the integration of metal sections with non-metal sections. Surface finishing operations, such as polishing and anodizing, are generally performed after the metal sections are secured to the non-metal sections. This means that contaminants from these manufacturing processes can get trapped within gaps between the metal and non-metal sections. These contaminants can leach out of the gaps and get trapped within the anodic oxide coating, eventually interfering with the anodic oxide coating coloring process, resulting in non-uniform coloring and cosmetically unappealing defects of the anodic oxide coating.