Butyl Tape & Caulk:
Polyisobutylene cross linked Butyl retains adhesion and elasticity in weather
extremes of -60̊F to +212̊F for prolonged periods, is highly water
resistant, and because it never hardens completely, it’s often used for sealing joints/seams where the joints are subject to shear. When used with a caulking or backer rod, Butyl also fills larger joints well.
Butyls are messy, and they generally have an unattractive tar-like appearance.
Many boat manufacturers (C&C etc) used butyl tapes between Decks & extruded Toe-Rails. Too bad.