Understanding Marine Sealants: They Are Application Specific

Over the winter, a many-thousand pound fiberglass, wood or metal shell is held in position by only a few plywood pads, two trailer bunks or an assortment of rubber rollers. In the spring, many of our boats are lifted by two slings into the water, putting enormous pressure on the sides of the hull, causing it to flex and shift. While in the water, the pounding of waves, straining of rigging, driving rain and UV exposure are the norm. It’s amazing that boats hold up under that abuse.

To exacerbate these problems, we’ve created boats from many types of materials that must work together in concert: steel is bedded to wood and fiberglass, rigid porthole frames are set into flexible cabin tops, plastic bushings and through-hulls are used extensively, bronze and copper are set at right angles to wood grain — and this is on top of the flexing, twisting and pushing forces that are constantly at work on a boat.

Bonding Agents

Boat builders, shipwrights, marine techs and owners reconcile all of these contradictory actions in a number of ways — but one of the most overlooked and undervalued is the correct use of sealants. (Note: We’ll use the generic term “sealant” to describe the material used for bedding, bonding and sealing. Another common term is “caulking.”)

Hundreds of years ago, gaps in planking aboard wooden clippers were filled using cotton, hemp and tar. Over the years, we’ve made strides in understanding how wood shrink and expands and the best methods to compensate for these changes. The era of fiberglass, carbon fiber and Kevlar has seen many chemical sealants become more sophisticated and specialized. With the number of options available on the market, how do you decide which to use, and when?



The Use of These products is Based on a Few Factors

• The purpose of the repair: Are you bedding, sealing or bonding?
• The location of the repair: Is it above or below the water line? Is it in a wet or dry area?
• The nature of the repair: Should the sealant remain flexible or are rigidity and strength required?

Different Jobs Require Different Sealants

Bedding: The process that seals out water when installing hardware. Any time a screw or bolt goes into wood or fiberglass, it should be bedded using a sealant.



When bedding hardware (such as putting a screw into wood or fiberglass) the purpose is two-fold: First is preventing the intrusion of water that would rot the core of the material the screw is entering. Second, it provides a buffer between the two materials as they flex and move at different rates (think of stainless-steel deck hardware against fiberglass — the fiberglass flexes much more than stainless steel).



Bedding compounds like 3M 5200, 4200, Sikaflex 291 and 292 are good choices

Sealing: Similar to bedding, it is the layer between two parts. The seal may be flexible or rigid. This is done around hatches, portholes, through-hull fittings and deck hardware. The use of a gasket material is also “sealing.”

Sealing comes in two types: above or below the waterline. Sealing is necessary in areas where solid hardware isn’t necessarily used — when installing a through-hull or a hull-to-deck joint, or between the planks of a wooden boat. Sealing is also done around portholes and hatches.

Sealants like 3M 5200, 4200, Sikaflex 291 and 292 are great for underwater usage. For above the water (on decks) marine-grade silicone products like Butyl tape, 3M Silicone and Boatlife Life Caulk. If an epoxy is needed to combine the benefits of bonding and sealing, West System’s G-Flex is an option

Bonding: Adhering two parts together. Also called gluing. Often rigidity and strength are called for.



Bonding can be done above or below the waterline for scarf joints, permanently securing hardware, or as a permanent adhesive. Products like wood glue, JB Weld and two-part thickened epoxies (like West System’s Six10) are good choices depending on the application.



Other factors that will affect your choice of sealant are: ease of application and clean-up, price, the flexibility of the cured sealant and its ability to be taken apart or repaired. For example, 3M 5200 is an excellent sealant below and above the waterline, but is difficult to remove once applied. Other factors such as color availability, cure time, ultimate strength and price.

A Few Tips To Make the Job Easier

When sealing around exposed edges, tape is your friend. Use painter’s tape to edge off the areas that you don’t want the sealant to adhere to, but make sure to leave enough of a layer or edge for the sealant to do its job.



Press the sealant in place — don’t rely on a surface layer — use a rounded tool or a gloved-finger to press the sealant in any joints, spaces or gaps.



Keep nitrile gloves, garbage bags, acetone and rags handy for cleanup. Acetone works well when removing caulking from fiberglass and gelcoat surfaces. Sealants tend to get into the grain of woods and can be quite difficult to remove so cover these areas with paper and tape while working. Finally sealants get everywhere on clothing and skin. Wear clothes that can get messy and cover your skin.

Andrew McDonald is the owner of Lakeside Marine Services — a boat repair/maintenance firm based in Toronto. He has worked in the marine industry for 12 years and is a graduate of the Georgian College “Mechanical Techniques - Marine Engine Mechanic” program.