Best Below the Waterline Sealants

When it comes to boat building, you need to make sure you use the best sealants for the job. Especially if the application is below the waterline, you need a product that will work long term. 

Why Below the Waterline Sealants Are Important  

Whether you are in a speedboat or a pontoon boat, there is nothing like cruising through the water on a cool summer day. If you are a boat builder, you need to make sure you use the right adhesives and sealants, or your customer’s beautiful day on the water could quickly turn into a nightmare.  

There are a wide variety of marine sealants on the market, but while they may be used on boats of various kinds, they are not all made to be submerged in water. Many products are not meant to be below water, and if submerged for a long time, they will not be able to hold their bond long-term. Eventually, it could lead to bond failure, and you can bet your customers will not be happy and the reputation of your company will suffer as a result.  

You need a sealant for below the waterline so it will create a watertight seal. Always check the product’s datasheet for this designation or speak with an adhesive expert about the product in question to confirm.  

Best Marine Sealants for Below the Waterline   

Below the waterline marine sealants can be used for a variety of areas on a boat including for through-hull fittings, transoms, center consoles, or bilge areas. You may also be looking for something specific to fit into your production process such as a longer working time or a sealant that can be painted over. We want to help you find the right product for your need.  

Check out our recommendations for some of the best below the waterline sealants for various uses.  

3M 5200 

3M 5200 Marine Adhesive Sealant

This product is a 3M below the waterline marine sealant, but it also is versatile enough that it can be used above the waterline as well. With a longer open time, it is perfect for large surface area applications including with through hull fittings, under molding, deck housing, and more. It can absorb shocks, vibrations, shrinking, swelling, and structural movements while still maintaining a strong bond.  


300 ml Cartridge of Sikaflex-291 Marine Adhesive

Generally resistant to both fresh water and salt water, this product has various uses in the marine industry. Not only can it be used for joint seals, but also it has several interior sealing applications. It is vibration resistant, weather resistant, low odor, and noncorrosive. It also has the added bonus of being able to be sanded and painted for a more professional final look.  

Bostik 920 FS 

300 ml Cartridge of Bostik 920 FS Marine Adhesive and Sealant

This below the waterline sealant can also be used as an adhesive above the waterline. It is compatible with a variety of marine substrates including wood, metal, FRP, and most plastics. It is also tear and abrasion resistant with a high elongation and a fast set time. Common applications for this product include use with hull seams, undermoldings, keel joints, and more.   

Boats are expensive and your customers want to know that they are getting their money’s worth. If you cut corners and use the wrong adhesive for the wrong application, this could spell trouble for both your customers and your business.

Instead, take the time to pick the right product for the job, and if you need more help, our adhesives experts are here. They can offer suggestions as well as help you problem solve when it comes to your specific needs. Just contact us today 


Related Products

Ask a Question

Have questions? Our team of adhesive experts can help. Submit your questions and we'll get you answers right away. We're here to help.