You don't need a uv resistant sealant
. The caulk will be shielded from UV by the chainplate if external or by the cover plate if deck
and Polysulfide have adhesive
qualities and should hold down cover plates actually better than sillycone. Use 5200 if you want to be sure the plates will stay down. You can bed
the chain plates themselves in butyl which I'd reccomend and the cover plates in whatever. Why can't you screw the cover plates down?? If the screws are stripped, clean out the existing caulk by drilling slightly oversize to be sure they are clean. Fill the screw holes with thickened epoxy
using a bit of straw or wire to be sure the epoxy
has no voids or you could jam a small bit of hardwood in the hole with the expoxy to give a screw more bite. Drill and tap for machine machine screws or drill for sheet metal screws to hold the plates down.
Dow 795 is sillycone. Granted it's a much better behaved sillycone but still sillycone. I'm not particularly fond of sillycone and looking to do gross bodily harm to the idiot PO's who used sillycone on boats I've bought. Sillycone belongs in Boobs not boats and I'm not even sure about that.
Use Butyl tape, polysulfide (LifeCaulk), or Polyurethane
(3M 5200/4200 and others) to bed
the chain plates. 5200 isn't a permanent sealant but you'd think it was if you ever have to remove something bedded with it. Probably acceptable for chainplates but wouldn't use it on anything that I had even the slightest inkling that I might need to R&R in the future.
Used GeoCel to seal some trim, etc on my house and wasn't impressed, wished I'd used Life Caulk.