Choosing the Right Marine Sealants

Using the right marine sealant for installations or repairs on a boat is essential. The marine environment is a harsh one, and a half-baked repair just won't last. 

Marine sealant leak protection

Choosing the right sealant may be the deciding factor between a boat floating or flooding! Not only will the correct marine sealant safeguard against leaks, but it'll make the job easier and the finished work will be durable and dependable.

Types of Marine Sealant

The first step in choosing a marine sealant is deciding which type of sealant will be the most suitable. 

Marine sealant comes in four main types:

Silicone

One of the most common sealants, highly versatile and very easy to use, but has the weakest adhesive properties.

  • Forms the weakest adhesive bond
  • Highly resistant to damage from heat and chemicals (bleach, fuel, solvent)
  • Compatible for use on wood, metal and all plastics

Polyurethane

Another common sealant and the one with the strongest adhesive properties.

  • Forms the strongest adhesive bond
  • Easily damaged by chemicals (bleach, fuel, solvent)
  • Compatible for use on wood, metal and some plastics, except polypropylene and Lexan

marine sealant adhesives

Polysulfide

A rubbery, versatile sealant that is easy to work with, although not as pliable as silicone.

  • Forms an average strength adhesive bond
  • Highly resistant to damage from chemicals (bleach, fuel, solvent)
  • Compatible for use on wood and metal only, but not on plastics

Polyether

The newest type of sealant available on the market. Polyethers combine good adhesive qualities with great resistance to water and chemicals.

  • Forms a strong adhesive bond
  • Highly resistant to damage from heat and chemicals (bleach, fuel, solvent)
  • Compatible for use on wood and metal and with some plastics, except ABS and Lexan 

Marine Sealant and Marine Adhesive

Marine sealants are also referred to as marine adhesives or adhesive sealants. This is because marine sealants create an adhesive bond to the surfaces they are applied to, as well as creating an airtight and/or watertight seal. 

wet boat deck marine sealant

The extent of a marine sealant's adhesive and sealing properties create a trade-off:

  • Marine sealants that are strong adhesives do not create the best seals
  • Marine sealants that create good seals do not provide strong adhesion

Silicone sealants provide the weakest adhesion to the surfaces they're applied to, but make great gaskets and seals. On the other hand, polyurethane sealants generate the strongest adhesion to surfaces, but do not make very effective seals.

Choosing the Best Sealant Based on Adhesion

For jobs such as repairing deck joints and hull joints that require a very strong adhesive bond, polyurethane sealants are the best. 

Marine sealant wooden boat deck

Products such as 3M's 5200 polyurethane adhesive sealants generate around 700psi of adhesive force, making them the strongest on the market. However, a virtually indestructible bond is not always the best solution. For instance, fittings bedded into the deck will need to be repaired or replaced from time to time. Therefore, using a "permanent" adhesive such as polyurethane will make the removal very difficult and damaging to both the fitting and the area it was bedded to.

For jobs such as bedding deck fittings that require a strong but not permanent adhesive bond, a polyether sealant or a lower strength polyurethane sealant may be a better choice.

Flexibility and Sealing

To form a watertight or airtight seal, the sealant must remain somewhat flexible and elastic once cured.

A soft, rubbery sealant that can be compressed to fill every last gap between two surfaces makes an ideal seal, whereas a rigid sealant is less able to compress into the voids between two surfaces. 

Marine sealants that remain flexible and pliable after curing create the best seals.

In addition, a sealant that retains its elasticity after curing retains its bond with sealed surfaces when they move around. 

fiberglass boat deck

A stiffer, more solid sealant cannot move with the surfaces, and so the sealant shears from the surface and the adhesion is broken. Marine sealants that keep their elasticity after curing create the best seals between surfaces that experience slight movement.

Choosing the Best Sealant Based on Flexibility

For jobs such as sealing a teak deck, polysulfide or polyether sealants are best because they can flex and move with wooden boards that expand and contract in changing weather conditions. 

Marine sealant wood deck

Both sealants are compatible with wood, and are excellent at resisting damage from deck spills. For jobs like sealing through hull fittings, silicone sealants are a good choice. Although silicone doesn't provide much adhesion, it creates a very effective seal. Once the silicone has cured, the through hull's securing bolts can be tightened to provide the necessary adhesion. In doing so, they compress the silicone sealant into every gap between the fitting and the hull for a watertight seal.

Curing Time

The time it takes from the initial application of a marine sealant until it has fully cured varies among the different types of sealant. Some sealants take several days to cure, whereas others cure in just a few hours. 

boat deck marine sealant

Because some sealants are specially formulated to cure quickly and others to cure slowly, knowing which type of sealant cures faster is a challenge. Consult the manufacturer's instructions to determine the actual curing time of a marine sealant. Also, keep in mind that some marine sealants used in hot, dry conditions cure faster than sealants used in cold, damp conditions. Polysulfide and polyurethane sealants cure faster when wet, so be mindful of atmospheric conditions when choosing marine sealant.

Choosing the Best Sealant Based on Curing Time

The longer the sealant needs to cure, the longer the boat needs to stay inactive. If you're hoping to complete a maintenance job one week and use the boat the next, the sealant's curing time will need to be less than 5-6 days. 

Marine sealant wood boat deck repair

If a boat is not expected to be used for a longer period of time, jobs with slower-curing sealants can be undertaken. For jobs like replacing a wooden deck, a compromise should be reached. Because the deck will be walked on during replacement, the sealant needs to cure relatively quickly. However, replacing a wood deck requires the wooden boards and the sealant to be "worked" for a considerable period of time, so a sealant with a curing time somewhere between slow and fast may be your best bet.

Compatibility with Different Materials

Not all sealants are compatible for use with all the materials found on a boat:

  • Silicone sealant is compatible with wood, metal, fiberglass and all plastics
  • Polyurethane sealant is compatible with wood and metal, but not with some plastics, including polypropylene and Lexan
  • Polysulfide sealant is compatible with wood and metal, but not with plastics
  • Polyether sealant is compatible with wood, metal, and most plastics, but not with plastics like ABS and Lexan

Choosing the Best Sealant Based on Compatibility

Because of the vast range of marine sealants available, there are no solid rules as to which is best at sealing a particular material. 

marine sealant wooden deck

For example, although polyether sealants are compatible with some plastics, they're harmful to others and should not be used on them. Always consult the manufacturer's directions to determine what a particular sealant is or not compatible with.

Putting It All Together

With so many variables to take into account — flexibility, curing time, adhesion, compatibility, etc. — you'll probably have to make certain compromises when choosing a marine sealant to best suit your needs. Use this guide and take note of the manufacturers' recommendations and directions to find the perfect sealant for the job!

 

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