sealing openings and equipment in outer walls.

For what you are doing I would just use the Butyl tape and trim off the excess once you put the screws in. It will not be exposed to UV light so I would not worry about that. Butyl rubber also comes as a caulk. When I worked at Sikorsky that is what we used to seal the airframe structure to prevent water leakage. It also has some adhesive properties but is primarily used as a sealer.