Beneteau F310 ~ APOTHIC
Join Date: May 2009
Location: Ontario, NY (PYC)
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It depends heavily on what the joint was initially sealed with. If it's butyl, then a little scraping and some Mineral Spirits will solve world hunger. If it's 3m 5200 you're in for a world of hurt.
Typically, doing this job "right" entails removing the fasteners from the joint, prying it apart carefully, scraping, cleaning with solvent, and then re-caulking it with an appropriate material.
I tend to think that when doing this job (as described) it's one of the few places where I would go with 4200 rather than butyl, but I recognize there are other viewpoints. I was able to squirt the 4200 into areas I would not have been able to lay butyl when I did my boat's hull-deck joint. However, if you can get butyl in, it's the way to go.
Either way, you want to clean out more than just the outside visible edge - you want to get into the joint to some degree to clean it and re-seal it in an area where the decks will push together to form a gasket. Anything less might help for a little while, but is not likely to be a long term fix.
Beneteau F310 ~ Apothic
PYC, Lake Ontario