After owning 2 Grand Banks over an 18 year span, I have not found any trick to prevent moisture getting into the joints. What I did discover is that the process of covering every piece of beautiful bright work with canvas does nothing to prolong the life of the finish. I have found that leaving it uncovered allows moisture to evaporate quickly and the finish, FWIW, lasts longer for me. The day I threw all those yards of canvas into th dumpster and relieved myself of that arduous task was a happy boating day. In case you are wondering, we are full time cruisers and our bright work is exposed to sun year round.
Howard