What They Do
Sealants form a watertight and airtight seal between two or more surfaces. Some sealants provide an adhesive bond (often supported by mechanical fasteners); others are used to isolate one surface from another against electrolysis, vibration or noise.

How They Work
Unlike glues, which usually cure as a relatively brittle solid, sealants cure to a tough, flexible, rubbery consistency. They adhere to surfaces to prevent water or air from penetrating. Because sealants are permanently flexible, they can withstand some movement of the surfaces to which they are bonded without losing adhesion. In fact, some products have such a high bonding strength that they may as well be used for eternal bonds. Curing time can range from hours to a week or more. Some sealants cure most rapidly in the presence of moisture.

What to look for
More than anything, the application and the kind of materials you plan to bond will influence what type of sealant you need.

• Above or below the waterline: Sealants that use talc or similar water-absorbing materials can fail if immersed for long periods of time, therefore they are not suitable for underwater application. Silicone-based sealants used to fall into that category too, but many of them have been reformulated for underwater use.
• Temporary or permanent bonding: Some products, like 3M 5200, have a reputation for secure bonding that should not be taken lightly. We’ve seen a 10,000 lb. keel that was put on with 5200 that held so strongly, it practically delaminated the hull during an attempted separation. Use compounds with less tenacious adhesive qualities if you know that you’ll have to remove the item at a future date, e.g. when bedding deck fittings.
• Paint or no paint: Some sealants can be painted, while others resist paint adhesion as if you were trying to paint a candle. Check the product description before you assume it will hold paint.
• Curing time: Despite many advances in the development of marine sealants one thing has not changed: They all require various periods of time to cure properly. Some materials may take several days to cure, providing lots of working time, but also potentially prolonging a haul-out or confining you to the dock over the weekend. Make sure you plan ahead. Even if you are able to use the boat, you could easily damage the bond depending what part of the boat is affected. On the other hand, moisture in the air or dew on the decks may accelerate the cure of products which use anhydrous catalysts, like polysulfides.

Comparing Compounds
Polyurethane: Recommended for permanent bonding because of its enormous adhesive strength. It is good for hull/deck joints and bonding thru-hull fittings but incompatible with ABS and Lexan. Do not use PU-based sealants if the bonded items might have to be separated in the future.

Silicone: Versatile, quick and easy to use, elastic and highly resistant to chemicals, silicone is excellent for isolating dissimilar metals. It is not as strong in adhesive strength as polysulfide or polyurethane but is compatible with plastics.