How to Choose the Right Sealant
Here’s a scene that plays out in the boatyard from time to time. A work order calls for rebedding a leaking porthole. After all the fasteners are removed, the hardware remains firmly in place. Some coaxing with a putty knife and some heat fails to free the porthole from the surface. At this point we can identify our foe. It’s a familiar antagonist known commonly by a number: 5200 (officially it is 3M™ 5200 Marine Adhesive Sealant). Out come wedges and hammers and solvents and curses and grunts. After an hour or two of struggle, the porthole finally lets go of the cabin side, but not without breaking off chunks of gelcoat and fiberglass. This unfortunate scenario begs this question: If the porthole is held in place with stout stainless steel machine screws, why does it need an adhesive to hold it in place?
UNDERSTANDING THE CHOICES
First you must understand what you are trying to do. If you are trying to hold a fitting in place, you need an adhesive. The hullto-deck joint would be one example; a fiberglass-mounted pad attached to a cabin top would be another. And if you want to hold it in place and keep water out, you need an adhesive sealant.
If you are trying to keep water out of a seam or a joint, you need a caulk. Caulking takes place after the item has been installed. Filling the seams between teak planks on a deck calls for caulking, as does covering a joint between a cabin top and a cabin side with a bead smoothed over the seam.
If you are trying to keep a fitting from leaking and/or to keep water from standing between the fitting and the mounting surface, you need a sealant. Examples include a ladder base mounted on the cockpit sole or a porthole mounted on a cabin side.
The sealant should be applied liberally so that you see “squeeze out” of excess material around the entire perimeter. Two-inch-wide tape helps keep the excess compound from making a mess.