Choosing the Right Marine Sealants

The right sealant for installations or repairs on a boat is crucial, as the marine environment is a harsh one. Choosing the right marine 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. Here's a guide to help you choose the right marine sealants for your boat repairs.

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. It's highly resistant to damage from heat and chemicals,  and its compatible for use on wood, metal and all plastics

Polyurethane

Another common sealant and the one with the strongest adhesive properties. It gets easily damaged by chemicals but it's compatible for use on wood, metal and some plastics.

Polysulfide

A rubbery, versatile sealant that is easy to work with, although not as pliable as silicone. It forms an average strength adhesive bond and is highly resistant to damage from chemicals. Polysulfide is compatible for use on wood and metal only, but not on plastics.

Polyether

Combines good adhesive qualities with great resistance to water and chemicals. It forms a strong adhesive bond and is highly resistant to damage from heat and chemicals . Polyether is compatible for use on wood and metal and with some plastics.

Marine Sealant and Adhesive

Marine sealants are also referred to as marine adhesives because they create an adhesive bond and an airtight and/or watertight seal to the surfaces they're applied to.

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

• Strong adhesives don't create the best seals
• Good seals don't provide strong adhesion

Silicone sealants provide the weakest adhesion to surfaces, but make great gaskets and seals. On the other hand, polyurethane sealants generate the strongest adhesion to surfaces, but don't 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. Products that generate around 700psi of adhesive force are some of the strongest on the market. 

However, a virtually indestructible bond is not always the best solution. For instance, fittings bedded into the deck need to be repaired or replaced from time to time. Therefore, a "permanent" adhesive such as polyurethane makes 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 applied 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 stay 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 during movement. A stiffer, more solid sealant can't 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. 

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 thru-hull's 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, while others cure in a few hours. 

Because some sealants are specially formulated to cure quickly and others slowly, knowing which type of sealant cures faster is a challenge. Consult the manufacturer's instructions to determine the curing time of a marine sealant. Also, keep in mind that some 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.

Selecting 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 needs to be less than 5 days. 

If a boat won't 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 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 option.

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. For example, although polyether sealants are compatible with some plastics, they're harmful to others and shouldn't be used on them. Always consult the manufacturer's directions to determine a sealant's compatibility. With so many variables to take into account, you'll have to make certain compromises when choosing a marine sealant. Always consult the manufacturers' recommendations to find the perfect sealant for the job.