Behind it is an airspace that needs to be filled
Gotcha, missed the part about that void - figured the fill piece was just a backer for the outer piece.
I know what you mean - spaces like that on a boat are famous (infamous) for mold and mildew. We get a ton of rain as well and, with the weather starting to warm up, the hard surfaces in those spaces are now carpeted in mold bloom.
I guess my question is: how is the water getting in?
Despite the coating, is the skin still a little porous ? It would be interesting to see how Rosie's boat was doing, were she still around.
Is the water running down and wicking up the fibers from the cut edge ?
It seems that, with a truly watertight skin, the only moisture in that space would be the normal humidity humidity on the day it was sealed up.
Of course, water is insidious and works its way into anything.
Maybe the notched plastic trim one uses on drywall for curved outside corners ?
Looking forward to seeing how you fix this one - I'm sure you'll come up with something clever
