Let’s say you’ve got a plastic antenna mount to install on your boat and you’re wondering what type of caulk to bed it with. If the next place you find yourself is at the marine supply shop facing a bewildering selection of marine sealants and sealants wondering which one is the best for your project, well, you’re not the first person to be in this situation.

The best place to start is with silicone sealants. And the best thing to say about them is that they don’t have any place on the outside of a boat. For anything. While silicone sealants are easy to work, they have almost no adhesive qualities, and often peel up. If you’ve got an interior project in a galley or elsewhere that isn’t designed to keep water out of the boat, such as a galley or head sink, it’s OK to use.

A package of Boat Life Life-Calk.

Polysulfides and plastics don't mix, but are great for many other boat projects. Image courtesy of Boat Life

Polysulfide caulks, such as Life-Calk or 3M’s 101 sealant are known not for their adhesive qualities (though they’re not entirely “unsticky”), but for their ability to cure without shrinking and stand up to the elements over time. They’re great for wood, metal, glass, or anything else that isn’t plastic, because polysulfide caulks can break down the polymers in plastics over time. So, if you have a stainless port light, a new piece of wood trim, or anything else non-structural to seal, a polysulfide sealant is a good bet.

Plastic marine fittings and polyurethane-silicone caulks (such as Life-Seal) are, as Forrest Gump said, “Like peas and carrots.” That is to say they get along great together. If you have a plastic antenna mount, PVC port lights, a plastic thru-hull-type drain in the cockpit, or anything else of that sort, a polyurethane-silicone is a good way to bypass the compatibility problems of a polysulfide sealant. It’s also easy to work with and clean up without the use of solvents.

A tube of 3M 5200 marine sealant.

3M's 5200 is a super-strong polyurethane sealant meant for critical jobs. Image courtesy of 3M

Polyurethane caulks are the super heroes of the marine sealant world and if there’s a “Super Man” caulk, it’s 3M’s 5200. Imagine trying to exert 400 to 700 psi of force on a fitting to get it free from your boat, and you get the idea—it’s amazingly strong, sticky stuff (and with good sealant qualities, too). Sikaflex is another good choice. Polyurethane sealants create strong, waterproof bonds on fittings such as metal thru-hulls, anchor rollers, chain plates, tower mounts, etc. Use it anyplace that a strong, permanent waterproof seal is required.

There you have it. If there’s any ever question as to what you should use, be sure to ask the pros at your local marine shop. Now get out there and get sealing—the right way. You’ll be glad you did when the rain comes.