3M 5200 is about the best stuff on the planet. If you are sailing a Hobie, check the comp-tip for leaks. Most other leaks will take place at the base or masthead castings or at points where people add hardware without applying sealant underneath. As also mentioned before, take your mast (without the boat) and lay it in the water near a calm beach and look for air bubbles around where the leaks are. You will need to rotate the mast and tip it a little from end to end because if you have more than one leak, one couuld be letting the water in while the other lets the air escape.