I'm not familiar with an International Folkboat but how is the deck attached to the hull? I would be more worried why there is a gap there. For example if
the deck is attached with bolts and they have become loose, caulking wouldn't be the first thing on my list. Do other IF's have a gap & caulking there?
If there is no structural issues, then clean all the old caulk off and use a good quality marine sealant.
Marine Caulking and Adhesive Sealants 3M Sikaflex Boatlife