My favorite bedding compound for hardware is polysulfides, (3M 101 and LifeCaulk). That being said, they do no work with plastic, like hatches and ports. Closed cell foam tape works, unless the flange is weak, and it scallops when you tighten the fasteners. Polyurethanes work with plastic, but you will probably break the hatch if you try to remove it later, the stuff is tenatious as an adhesive. Butyl is a great choice.