On interior surfaces large aircraft use a product called Dinitrol. It
gets sprayed on at manufacture and is there for the life of the
aircraft. It keeps moisture from getting between the joints.

That's the stuff my A&P had me paint all over the spar (and adjoining
structure) when we performed the dreaded Service Bulletin 1006 on my
Cherokee 235, a few years back.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"