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Self-adhesive "butyl tape" is NOT ideal for sealing acrylic windows for boats.
Butyl Tape (Polyisobutylene-Isoprene) has long been used as a spacer material (gasket), draft & vapour barrier, and dielectric insulation on Automobiles, RV’s and Residential Windows.
These tapes (and others like silicone, polyurethane, polysulfide, or EPDM - Ethylene Propylene Diene Monomer () are compressible plastic materials used to seal any opening or junction of two parts, such as between the glazing and its frame.
Glazing tapes are generally self-adhesive, UV resistant, and easy to apply (to clean surfaces). Residential window installers use butyl tape between the thermopane panel and the wood frame as a weather barrier.
Because it never hardens and is sticky, it is able to maintain an effective SECONDARY seal.
Most have NO bonding strength, and must be used with frames and/or fasteners.
I’d prefer to use a UV-Resistant Urethane or Polysulfide Caulking, such as:
3M™ Auto Glass Urethane Windshield Adhesive 08696, a High viscosity, one-part, moisture curing urethane adhesive, that bonds auto replacement glass to car bodies.
Urethane Windshield Adhesive 08693, a Medium viscosity, one part moisture curing urethane adhesive designed for windshield bonding and stationary glass attachment.
Automotive Windshield Instructions (3M):