We swear by butyl rubber tape, having used it to rebed every piece of deck hardware on our sailboat. The stuff will stay flexible for decades, allowing it to continue to seal even if the attachments flex. It does not have any adhesive qualities, but you don't need that if you have good mechanical fasteners. As an example, we had a stanchion bent 90 degrees during an altercation with another boat during a storm, and even then it didn't leak. Not all butyl is created equal. We bought some from the hardware store that we wound up throwing away. The best stuff I've found is the type RC (Mainesail) sells through his Compass Marine website (link in one of the above posts). Order two rolls - it won't go bad and shipping is the same. Also, this particular tape is light grey, not black like the stuff they sell at auto parts stores. Clean up is really easy - any that oozes out after tightening down the hardware can be picked up by using a small ball of the stuff, which the squeeze out will stick to. In sum, it is all we use now for all above waterline hardware where we don't need adhesive qualities (which is just about everything).
