"Bob" wrote in message
...

Is there some kind of (liquid ?) waterproofing product avail. at HD or
Lowes that I can apply to a cardboard mailing tube to make it waterproof ?

Would like to use it to store a fishing rod over the winter in the
basement, and I would prefer that the tube not absorb any moisture.

Something is fishy here. Fishing rods are designed and built so as
to survive without harm exposure to the whole range between 100%
humidity (total immersion) and the lowest natural atmospheric humidity.
There is no need to shield rods from ambient humidity unless local
conditions would also promote mould/mildew on the rods.
--
Don Phillipson
Carlsbad Springs
(Ottawa, Canada)