It's been ages since I did mine. (I only did one) I think I used Dow 795, but don't quote me. I'm sure the experts will chime in with more authority. What I do remember is that I sealed the exposed core of the opening with epoxy (it wasn't done in the build)

 

If it’s compressed with screws or bolts use butyl. I use it for everything. If it needs to be glued, maybe 4200?

 

Gluing something one might one day wish to remove is not a really good idea. Caulking or bedding compounds are much better suited to most jobs on a boat.

 

I used 795 to bed the glass into the frames and 4000 to bed to cabin sides on my Mirage with great success.
-CH

 

Dow 795

 

Hi Friends, I just completed some of this project. I used Sika 291. I would avoid 3M 4000UV due to it's history of failure. Practical Sailor: "3M UV4000 failed completely and became gummy during the fifth year of testing." 3M says that it has been reformulated. But still?

 

Correct me if I’m wrong but the issue is that the inside and outside layers of the boat the frame of the port light has to span aren’t rigidly connected so compression of the inside and outside parts of the frame doesn’t generate much compression on the boat surfaces.
One type of repair I’ve seen is inserting a bead of “caulk saver” which looks like a round hatch seal gasket leaving enough room for thickness epoxy which will then allow for compression. In this case butyl tape would work.

 

I did my Newfounds with butyl 2 years ago. No leaks at all. Look at their instructions for sealing. Seems as though a form of neoprene tape was used in part of the process.

 

I live and die by OSI Quad Max. It's an exterior 50yr structural urethane adhesive caulk/sealant. Permanently flexible, non-staining and algae resistant, bonds to anything, bonds and cures under water, available in a large color palette and is paintable. Removal would be your only other consideration. I use it for everything.. Basic colors are available at all home stores.