Best way to keep the coax watertight :

Scotch 33 + Electrical Tape and Plastidip (http://www.plastidip.com/) around the areas that are exposed. So in the case of the center post, apply the tape around the open ends of the coax and
then apply the plastidip around the coax and coating the element taps (except the ends that the elements will attach to). Apply it with a popsicle stick or similar applicator.

If in the future you want to remove the plastidip, you can easily peel it off.

Source : OM Jason, N8XE (hex-beam@yahoogroups.com)