Ahoy,
We're ready to apply the polysulfide to the cabin decking. I had thought we could use some form of exterior silicon, but apparently not. So polysulfide it is.
Is Boatlife the brand to use? Apparently, in bulk you pay a little over $13.00 per cartridge for no other reason than anything nautical must cost much more. It's the law of the seven seas.
What else do you need? What kind of tape?
Polysulfide are typically used in deck seams, but your seams are sealed so you have some options. 3M 4000 UV is a polyester and generally cheaper than the polysulfide, though you'll not use as much as you think (unless those seams are real deep) and it can be removed much better than 5200 (which is also cheaper than the polysulfide). A polyurethane like 5200 or 4200 can be used, though will make repairs more difficult if the need arises. Home Depot use to sell 3M 5200 pretty cheap (under 10 bucks), making it a good deal, I don't know if they still do.
Boatlife Sandable Silicone Sealant is an option, but it's near 20 bucks a cartridge. Personally I'd hate to recommend a bathroom tub and tile type caulk and have it peel out of the seams after a year. I do know of some builders that do use some of these hardware store sealants with good results, but they don't have desert bound vessels either.
Sikaflex 291 is as costly as the polysulfide, though would work fine.
In the end you'll want sandable, reasonable UV stable goo, that can be removed without ripping bits of decking out in the process, if the need comes along.
Just be grateful you have the option of a single part goo in a tube. Traditional seam sealants are two part, come in quart cans (or larger) and a big mess to work with.
Wait the maximum length of cure time (usually a week) before you sand the seams level, trust me on this one.
Black is traditional, though white looks cool and adds contrast to a dark deck.
I use clear plastic packaging tape, but most use the blue 3M/Scotch painters tape or their green "fine line" tape. I use the packaging tape because it's thinner, is very cheap, not much will stick to it and has a very crisp edge, which the "crape" tapes don't, sometimes permitting goo or paint to ooze under the edge.