Matt-
There's only one real way to seal Islander ports: Break 'em out and replace them. The originals are still available from Mark Plastics (somewhere in Cali) who still swear they should be installed with silicon seal (not my concept of a great sealant). In order to get the old ones out on an I28, we had to pop the glazing them "implode" the rims, collapsing them in order to break the bond of the remaining original silicon. Lots of cleaning to get that all off, lots of generous rebedding to install the new ones. I just don't trust silicon sealant of ay kind to make a long term (20 year) seal for that type of application, I'd rather find out what's compatible with the plastic and use a BoatLife or Sika or 3M higher-tech product.
On the chainplate, that's serious too. It can result in loss of the bulkhead especially if the bulkhead is staying damp behind the teak trim strip against the hull. Usually to reseal a chainplate, you slack the
rigging (use a halyard ot the rail as a temporary stay) and unbolt the chainplate from the bulkhead, then lift it out completely. Clean out the spot that penetrates the deck, if there is any rot, clean it out and replace with
epoxy. Otherwise, seal with
epoxy, leaving a gap around the chainplate itself. After the
epoxy has dried, fill the gap with your choice or good bedding compound when you reinstall the chainplate. Another matter of personal religion <G> to pick the bedding you think will be most durable and FLEXIBLE since it must flex with the deck or it will tear up and leak. I'd also suggest putting a piece of neoprene or other durable synthetic rubber over the chainplate before screwing down the trim plate--so that compresses and forms an extra seal.
And last of course reconnecting the
rigging. Neoprene and other sheet rubber can be obtained from Grainer and McMaster online or their local stores if you have no local source.
A good way to make sure you've gotten ALL the leaks, is to close up the boat, including all vents, and use a piece of corrugated cardboard or thin scrap instead of one companionway board. Stick the
exhaust from a shopvac or leaf blower in through that one board, and pressurize the boat.
Now, throw buckets of sudsy water all over the deck and fittings. (Slippery on deck, careful!) You will find that the boat "blows bubbles" quite literally from the source of every leak. No matter how the water travels into the boat--those bubbles will show you the point of entry that needs to be resealed.