Effect of Polysulfide and polyurathane on plastic ports

[mepsnbarry]

I just thought I'd re-open this question, since it's never really been resolved.

We are re-installing some Beckson plastic-framed portlights, and the manufacturer says "only use 100% silicone." Since silicone skins over in 5-10 minutes, our surveyor says the beginning of the bead will skin over before you're gotten the stuff applied to the entire surface. It is destined to fail.

So will polysulfide actually degrade a Beckson plastic portlight?
What about polyurethane?
Will a polyurethane/silicone blend, like Boatlife's Life Seal, work instead?
Lastly, what about butyl rubber? It works OK for temporary fixes, but can it be installed well enough for an offshore, water-tight seal?

[lbdavis]

Check out this report from our esteemed member, Maine Sail.

Good enough for Maine Sail, good enough for most.