Progress in hydrogen fuel powered systems has been propelled by the implementation of secure, reliable, and cost-effective hydrogen storage and transportation technologies. The fourth category, distinguished by a polymer liner serving as a hydrogen diffusion barrier, fully encapsulated within a fiber-reinforced composite to bestow structural integrity, has garnered substantial attention from the automotive industry due to its lightweight nature and rational manufacturing process. The method of rotomolding has sparked interest among manufacturers due to its capability to directly bond the metallic component to the polymer substrate, specifically the liner, thus negating the need for welding and its attendant imperfections. In fact, a pivotal facet of fourth-generation hydrogen storage systems revolves around the interface connection between the polymer liner and the metallic boss, posing as a structural Achilles' heel. For the study's purposes, a scaled-down demonstrator was fabricated using rotomolding in which a nozzle-liner interface mimics the boss-liner interface of the actual system. This demonstrator was designed to facilitate the mechanical characterization of the interface under quasi-static and fatigue loading. The thermal cycling phases of rotational molding and the surface treatments undertaken have been optimized in order to enhance direct adhesion within the metal-polymer interface. This study commences by assessing the efficacy of two treatments (sandblasting and flaming) applied to the aluminum nozzle surface. Subsequently, we explore the adhesion microstructural and mechanical characteristics of the treated nozzle onto a medium-density polyethylene polymer (liner). Lastly, we delve into an exploration of the damage and fatigue behaviors endemic to the metal-polymer interface region. The obtained Wöhler curves disclose a linear trend for the metal-polymer interface. Moreover, the metal-polymer interface evinces heightened resilience against damage and fracture for sandblasted interfaces. This inquiry underscores the potency of innovative polymer-metal interfaces treatment in amplifying the reliability and robustness of hydrogen storage technology.