As an advocate of 5200, never use that on installation of an opening port. You want to use an archetectual grade of 100% silicon which can be purchased even at Ace Hardware stores or other places. Generally, Hunter used 3m product but the key here is what I said.
There is a trim ring on the outside that you will need to take off glued in place. I use to pry open with a scraper or thin blade scraper and worked around until all was off. Be careful when scraping not to gouge the fiberglass. Go inside and remove the screws keeping them for the installation of the new port or window. Again remove all sealant and you can use either Acetone or Alcohol at this point which ever you prefere. However, do not use any acetone after this but only alcohol as acetone will eat up plastics and craze lexan but alcohol will not. I use to go with alcohol only when replacing a port/window. Again remove all old sealant. Make sure the screw holes are not holed out and if so, repair them before installation.
Put the new port in for a moment to make sure the screw holes are aligned. Then apply sealant so it will ooze out thru the window and opening to insure a good seal between the port and deck opening. Make sure you also apply sealant on the threads of the screws. Clean off any excess.
Where the trim ring will go, I would lightly sand the gel coat area say with 320 grit wet and dry sand paper for good adhesion and sometimes I did that to the underside of the trim ring as well. I then applied sealant and put the trim ring on and again I would make sure there was enough to ooze out for a good adhesion and then clean off. Make sure you press the trim ring in place all the way around. Make sure the color of the sealant is black.
I hope this helps. Once I did not have any alcohol in the shop but remembered the wonderful peach brandy I made the night before and used that. It is called moonshine and it was some of the best. I use to deliver that to Warren Luhrs when he use to sail solo in races around the world. Oh, I did not charge for that. Moonshine has many purposes but the best is for sippin.
crazy dave condon