Wish I had an answer, but I can certainly feel the pain on this one. I spent decades building in the Northeast, and finding ANY evidence of failed PT lumber in normal residential applications is pretty unusual. Last winter I did some volunteer work at a state park in central FL. We repaired a boardwalk though a swampy area of a busy campground. The structure was all PT yellow pine and well built. The damage was extreme, with a collapsing walkway, and some of the joists degrading to the point that they were essentially composting and returning to the earth. The entire project was originally built as a boy scout project, SEVEN years previously.
Just an educated guess, but I think the heavy coating you discuss will be a disaster. It will encapsulate the top surface of the boards, blocking the ability of the material to properly dry between rain and high humidity cycles. Obviously the material dries by evaporation on the sides and bottom, but I would think it would be a fraction of the drying that takes place when the top face of the material is exposed to sunlight, and radiant heat from the sun.
Good luck with your project. We spend our winters in Fl. and I find the climate and related construction techniques and issues to be pretty interesting. That said, I'll stick with what I know, no termites, fire ants, elevations of a few feet above sea level, hurricanes, "lifetime" building products that rot into mulch in a few years, etc........We are in a big campground at the moment, and at least two employees spend most of their time hauling biomass off the property. They rake, shovel and saw at everything from grass to palm trees continiously, in order to literally keep the jungle at bay. I get tired just watching the battle, LOL.
