I like to use the liquid electrical tape on the joints before the shrink tube. It really helps keep the elements out AND inhibits corrosion as it seals it from the air.
Paint it on with the little brush and let dry, then heat shrink.
I once had some shrink wrap with a "sealer" in it. As it shrunk, this clear gel would squeeze out of it and it would seal water tight. I got it from guy in the electrical lab, but I don't know if it's available to us "common folk". I've tried looking for some ever since, but can't find it at any stores.
