Review on 1/2" 55Ft XHF 3:1 Waterproof Adhesive Lined Heat Shrink Tubing Roll Marine Grade Insulation Sealing Oil-Proof Wear-Resistant Black by Steve Yang

This has something akin to hot glue coating the inside, so when it's heated enough to shrink, that coats everything and seals it completely. It says it shrinks 3:1 and I'd say that's about right without measuring, and it's thicker when shrunk than I expected. Since it went on an outdoor cord, the thickness and good seal are both desirable. Won't know how well it holds up until it's been outside a while, but the initial results are certainly promising.