Someone mentioned in another thread they had a couple portholes leaking for years...
I learned the hard way... Never never ever put off re-bedding your portholes.
I just had my decks/cabin top painted. After also having what I thought was a couple portholes leaking for years I decided that since my portholes were out now would be the time to repair the dry rot underneath them. The more I dug into it the more dry rot I discovered (including a couple portholes I didn't know were leaking), extending to the plywood 'fascia' covering the underside of the fiberglass deck.
While I knew I had some soft spots underneath the portholes the only visible evidence underneath the fascia was the mahogany veneer starting to peel away in a couple places. Actually I just thought it might be cosmetic... the veneer glue failing after all these years.
What I discovered was the original 3/4" plywood fascia had virtually completely delaminated, nay disintegrated, coming out in 1/16" thick pieces, only held together and in place by the fasteners holding it to the underside of the deck and the handrail running the length of the cabin.
In researching what to use to rebed my portholes I came across this website (see 'installing new portlights').
http://www.pbase.com/mainecruising/boat_projects
He makes a strong case for NOT sealing anything from the interior side, just the exterior. Doing so may trap water (or divert it somewhere else) because it can't escape out the back. With my portholes being sealed on both the exterior & interior sides I think that is what happened in my case:
After years of leaking, water found a channel in the dry rot underneath a porthole and 'wicked' onto the fascia in the space between the deck and the plywood rotting it from the top down and thus no real visible evidence underneath. I'm not sure how but even my handrails still felt solid.
As well as scarfing new wood underneath the offending portholes I just had to have the entire fascia replaced with new 3/4" mahogany (veneer) plywood all the way from the aft (companionway) bulkhead to the V-berth, port & starboard. Fortunately, the bottom 1/8" or so was intact enough so as to serve as a pattern for the new replacement fascia. I have to say, it's looking good though :)
