Cut off one half of the tip of a plastic syringe with a sharp knife. Mix some slightly thickened epoxy, fill one of the plastic syringes, and run a bead of the epoxy mixture along each lap on the inside of the hull and along the keel joint between the ties, radiusing the joint with a Popsicle stick to make a fillet.
Mix up another 2 oz. of a non-sagging epoxy/low-density filler mixture and fillet the inside of the stems, using a tongue depressor as a tool with the proper radius. It's really hard to work up in the stem area, but don't worry, there's an easier way. If you have trouble filleting the stems, try pouring a stem plug. If you decide to pour a stem plug, don't worry about doing a great job on the stems; at this point all you want to do is get enough of the epoxy mixture in the stems to hold them until the stem plugs are poured. Clean up any extra epoxy mixture, especially in the stems where it will be hard to sand later, and double check the hull for twist, mis-aligned stems, or anything that doesn't look just right.