sealer for bronze port
I just went through this last winter--replacing and bedding 11 portlights in my boat. I did a lot of checking, including with several manufacturers, and found that professional installers use either 3M 4200 (polyeurothane) or 101 (polysulfide). (And I sail better than I spell........)
Do NOT use 5200--if you ever have to remove them, you will end up doing what I had to do to remove my plastic portlights--use a reciprocating saw. I used 101, and it has worked extremely well. One professional installer said--"I buy whatever is available at the time, 101 or 4200--both will work fine."