Why You Need it
Sealant "is a generic term, but many sealants have specific uses and characteristics, so they
must be chosen and used carefully to insure that they perform as needed. Most are about the
consistency of toothpaste and cure into a rubber-like state. Sealants are an important part
of boat construction, since their primary function is to keep water out of areas where it could
cause damage or become a hazard. A sealant can also do triple duty when threading screws into
fiberglass or wood. It will first act as a lubricant to ease the screw's entry, then seal the hole
and become a locking agent to keep the screw from loosening up.
What You Need To Know
Silicone and polysulfide sealants (used above and below the waterline respectively) are the
standard sealants for modern boats, but are being joined - and sometimes now replaced - by
urethane-based compounds. Silicone and polysulfide sealants that are specifically formulated for
marine use are still viable and reliable products when properly used, but urethanes outperform both
in many situations and have the advantage of being usable above or below the waterline. They have
much better adhesion than silicones and perform better in the face of vibration and stress. They
hold so well that fast-curing urethane sealants with somewhat less tenacity are now available
because customers were complaining that they couldn't get the joint apart when they needed to.
The
most important consideration in the successful application of any sealant is surface
preparation. Sealants don't adhere well to chalky or dirty finishes because they provide a false
surface for adhesion. Use an abrasive cleaner to remove the contamination before applying the
sealant, but avoid rubbing compounds, as they may contain oils or waxes. The surface must be
completely free of oil, wax, dirt, mold and old sealant, all of which can interfere with proper
bonding. Wiping the area with a solvent, such as acetone, should be the final step in surface
preparation.
The Final Word
Silicone sealant is an inexpensive and effective product for general interior and
above-the-waterline use, but be sure to only buy a marine-grade product. Other compounds are a
better choice for use where adhesion is important or below the waterline, especially when your
safety or protection of critical components is at stake. Use all of these products in strict
accordance with the manufacturers' instructions.