
Butyl rubber has a long history as a waterproof seal, and it works. The prerequisite is that it is held by a flange or other device. It never hardens, but rather spreads when squeezed. It will sag under its own weight as the piece lingers. So for boat fittings, screw heads, hatches etc. It's perfect. Unlike polyurethane and silicone sealants, it does not "harden" but remains flexible and removable.

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