I just replaced two Beckson portlights
, last weekend. You remove the trim ring first and cut the old port out. Then go buy a couple of those $2 razor knives, fully extend the blade and with the new edge shave as close to the gel coat as you can. This next tool is the greatest, it's made by 3M and called a Bristle disk 120 grit. Insert in a drill and clean remaining silicone off, careful to the stay withing the trim rim. If you go to far off don't worry with gelcoat
. The disk isn't that aggressive it's made with nylon and mine the light scruff marks buffed out very easily, but removes the silicone very fast. If you have Beckson portlights
they recommend using silicone like Dow 795 for the reinstall. I been dealing with silicone on this boat and this tool is great. The plastic razor blades don't to well for scraping the old sealant
off, but work great for trimming any excess after the install around the trim piece.