Quick Guide to Marine Sealants

The right marine sealant can make the difference between safely installing a thru-hull fitting or springing a leak below the waterline. With so many sealants on the market these days, selecting the best one for the job can be confusing.

We’ve covered marine sealant types before in detail, so we’ll keep this one simple. Here’s a quick guide to understanding marine sealants.  

Sealant vs Adhesive

Adhesives are merely bonding agents, while marine sealants create a bond that joins two or more surfaces and/or materials together. Sealants create a barrier that prevents a liquid or gas from passing through an opening or coming into contact with a surface. Marine sealants also differ from pure adhesives in that they remain flexible once they’ve cured, whereas adhesives tend to be solid when dry. This flexibility is what enables sealants to create an effective seal, especially when used on boats.

The Different Types of Marine Sealant

Polyurethane Sealant 

• Polymer sealant that offers the strongest adhesive properties of any marine sealant 
• Highly resistant to water but vulnerable to chemicals such as bleach, fuel and solvents

Uses

• Permanently bonding above and below the waterline fixtures
• Repairing deck joints and hull joints

Pros

• Very high shear strength and unlikely to break at a glue line
• Compatible with wood, metal and some plastics
• Can be painted

Cons

• Once bonded, surfaces are hard to separate again
• Can be damaged by chemicals
• Not compatible with some plastics, including polypropylene and Lexan

Adhesive bond: Strongest

Flexibility: Weak

Silicone Sealant 

• Synthetic polymer compound sealant 
• Versatile and easy to use 
• Remains pliable after curing 

Uses

• Gaskets
• Sealing thru-hull fittings
• Insulating electrical components
• Isolating one type of metal from another

Pros

• Easy to work with and manipulate
• Highly resistant to heat and chemicals
• Safe to use with all plastics
• Non-conductive

Cons

• Low adhesive properties
• Can’t be painted
• Leaves behind silicone residue that prevents adhesion and is hard to remove

Adhesive bond: Weak

Flexibility: Strong

Polyether Sealant

• High adhesive qualities
• Great resistance to water and chemicals

Uses

• Sealing teak decks and thru-hull fittings
• Bedding wood such as rub rails

Pros

• Easy application and clean-up
• Highly resistant to heat and chemicals
• Can be used with wood, metal, fiberglass and most plastics
• Can be sanded and painted

Cons

• Not compatible with some plastics, including ABS and Lexan

Adhesive bond: Strong

Flexibility: Average

Polysulfide Sealant

• Synthetic rubber sealant 
• Durable and versatile
• Can be used below and above the waterline 
• Resistant to chemicals such as bleach or marine fuel

Uses

• Sealing teak decks and thru-hull fittings
• Bedding wood such as rub rails
• Bedding tank vents and tank fills

Pros

• Easy to work with and manipulate
• Unaffected by water
• Compatible for use with wood and metal
• Can be painted

Cons

• Not suitable for many plastics, including ABS, acrylic, PVC and polycarbonate
• Not as pliable as some other sealants, so it shouldn’t be used in thin layers

Adhesive bond: Average

Flexibility: Strong

Sealant Curing Time

Another factor that differentiates the various marine sealants is the time each one takes to fully cure. Some sealants cure in a matter of hours, while others may take days or even weeks to cure properly. 

• Quick curing time: Allows decks to be walked upon without affecting recently applied sealant, but may prevent the sealant and the materials from being sufficiently worked in time.
• Slow curing time: Allows the sealant and the materials being sealed to be worked for longer, but prevents the boat from being used until the sealant has finally cured.

Like with any other product, read the manufacturer’s instructions before applying marine sealant to your boat.