Whether you live aboard, enjoy your boat for leisure or work, your boat becomes an enternal circle of labor and you will always have to maintain it. You've noticed those dirt trails, and water droplets forming around bolt heads or even an actual annoying drip, but now it's time to fix that leak the CORRECT way. Done right, it's one of the easiest, most satifying and most important jobs aboard, one that will not only keep your boat dry down below, but also prevent major structural damage. Done wrong and it can destroy your deck coring and end up costing a great deal of money and time to fix.

KNOWING THE DIFFERENCE

sealant table

A good marine sealant for bedding deck fittings must be waterproof, of course, but it must also be flexible, UV resistant, and, ideally, chemical resistant (fuel, bleach, and other solvents do find their way on deck occasionally). It should not be so strong that the deck hardware can't be removed if necessary, or so tenacious that it leaves a residue that prevents other sealants from adhering. From an aesthetic perspective, it should resist dirt and not age in an unsightly way. The table above summarizes how the various adhesives line up against these criteria.

An adhesive sealant can be used to seal fittings but also has adhesive properties so it can mechanically bond to certain materials. In short,  you could stick your friends bum to a chair using it, but the prank may take up to 24 hours as it can take a long time to cure.

A non-adhesive sealant will generally rely on the aid of a fitting, screw or bolt to create a waterproof seal.  With a normal sealant, you are essentially creating a waterproof gasket.  You can’t rely on normal sealant to hold anything in place;  you are relying on a method of fastening, for example; a screw or bolt.

 TYPES OF SEALANT

Silicone

It is said that silicone is an inert synthetic polymer compound and is technically part of the rubber family. You can imagine why its been a favourite in boat maintenance for some time. Its UV resistant, heat resistant, a great insulator and is ideal for creating gaskets. Its downfall is that it makes for a weak adhesive and for this reason sometimes is unpractical for use below the water line.

Hybrid

Hybrids are combinations of the best elements of existent sealers. The idea being that each type of sealant has it’s pros and cons so they just pick the pros from the best types. For example the strength of polyurethane with the UV heat resistance of silicone. They will bond better but aren’t as strong as polyurethane.

Polyurethane

Polyurethane sealants create an extremely strong mechanical bond when used on the correct surface. They are UV resistant and can be used below the waterline.  In the aviation industry on unpressurized aircrafts, windows are sometimes attached using adhesive sealant only.  This is a testament to the strength of modern sealants. It’s notable that polyurethane is not compatible with acrylic perspex and other types plastic glazing.

 

Polysulfides 

Polysulfides; a synthetic rubber, forms a strong mechanical bond and has an excellent resistance to UV, fuel and oil as well as good general corrosion protection. Polysulfide also makes a great electrical insulator and is resistant to vibration, shock, impact, and thermal changes. It can be easily painted.

Butyl

Butyl is a non-adhesive sealant normally found in the form of tape and has the characteristics of chewing gum, but can now be purchased as sealant. It’s great for serviceability and it’s easy to remove and re-apply as well as being a lot less messy than traditional sealants. It could last 20 years but it won’t look great. It’s susceptible to UV damage and can often look like a bit of old chewing gum. It’s not suitable for use below the waterline.

 

Don't just pick up any tube of marine sealant from your favorite chandlery and set to work. If you want to make sure that leak doesn't come back, take the time to select the best sealant for the job. While it may not be as much fun as playing with drills and bolts, choosing the right sealant is every bit as important as the proper technique to make that fitting watertight.

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