On Sunday, September 30, 2012 10:27:39 AM UTC-5, Leon wrote:
As Leon has said, UV protection is important.
I would add, though: You mentioned the old finishes haven't lasted for more then a few months, before they fail. A good marine finish should last longer than that, even when exposed to lots of sunlight. I highly suspect the high humidity has elevated the moisture content, within the wood, and this is having the effect on the doors' finish, much sooner than should be, even in that environment.
After you strip a door (or a few at a time), place it/them in an air conditioned environment, to dry out more so, then apply whatever marine finish onto it. There will be less moisture in the wood to contribute to the finish's failure, if mositure is, indeed, an excellerating enfluence.
Many folks, even in other climates, forget to seal the bottoms of doors, to prevent moisture from entering the wood. Do any of your doors have problems with the swelling of wood, at the bottoms of the doors, causing periodic jamming or dragging of the bottoms of the doors on the thresholds, especially on or immediately after rainy days? Have you ever noticed if any doors jam, when opening/closing, on the sides or top, on or after rainy days, ie., on/after extra high humidity days? If so, then this may indicate there is a moisture issue that may be contributing to the finish issues, you have described.
From what you have described, I highly suspect the doors may be "damp", to the point of having the applied finish fail, earlier than would normally be so. Dry out the doors, as much as possible, in an air conditioned environment, if possible, before applying the finish coats.