Structure and method of sealing a closure assembly onto the neck finish of a plastic pressure container

A plastic pressure container comprising:

• a main body portion defining an interior space;
• a finish portion unitary with the main body portion and defining a mouth in communication with the interior space, the finish portion having an inner surface, an outer surface and an upper rim proximate the mouth, the inner surface having an inner undercut defined therein proximate the upper rim, the inner undercut having a curved shape in cross-section, and the outer surface having an outer undercut defined therein proximate the upper rim, the outer undercut having a polygonal shape in cross-section, wherein the inner undercut defines a first depth relative to an inner diameter D2 proximate the mouth and the outer undercut defines a second depth relative to an outer diameter D3 proximate the mouth, the second depth being different than the first depth;
• a closure assembly mounted to the finish portion in engagement with both the inner undercut and the outer undercut;
• and
• contents in the interior space.