Joe:
I know this sounds crazy but we have some of the same prob down at the beach. So use the same remedy we use for spices, salt etc. Throw some rice or talc in a a large baggy put it over the seat down to the seat post and tape it air tight. I know it is a pain but it will keep it dry. Or if you are really well to do (Energy expenses) put a space heater in and dry the joint.
Jack (Mr. Practical ) Gabus
Laguna Beach, CA
jack@shermangabus.com