SJ23 Tech Tip G01, (Updated 2015-10-26) Bob Schimmel


About Sealants and Elastic Adhesives.
(Dedicated to my son Marty who could lay down a perfect bead of sealant on the first pass and
who "modified" his paint ball guns to perfection before his life was so tragically taken from him. RIP).

INDEX - Caulking, Sealant, Elastic Adhesive, Rigid Adhesive, Epoxy, Fittings.

"Water, the universal solvent.  Sooner or later it will dissolve everything.
You can't stop it, you can only redirect it."
Marty P. Schimmel, May 14, 1978 - Aug 19, 2009.

Once you accept this fact you are on the road towards making the correct decision to do something about your leaks.  I hesitate to say solve your leaks because mankind has yet to create a goop that can stop the flow of water forever; although butyl rubber comes close.  Even the best of goop will eventually break down with water, loosing its adhesive bond.  Very frustrating but a fact of life.  If you repair your own boat you should understand the properties of the various types of "goop" available.  There are three categories of goop; caulking, sealants and elastic adhesives. 

CAULKING - You can eliminate caulking for a boat right off the bat.  There is absolutely NO place on the your boat were this stuff can be used effectively, even if your boat is on display in a museum.  If someone invites you for a ride and you discover caulking onboard, suggest an alternative like going to the nearest pub.  Enough said! 

The other two categories of goop are sealants and adhesives.  They have many applications and to use them effectively you have to understand the properties of each, the material it is designed to adhere to, the exposure to the elementsand the application of the joint.  In some cases adhesives and sealants can be interchanged with good results, though generally indoors only.  Just when you think you have all of this figured out, Murphy's Law kicks in with less than desirable results!  So, read the label carefully and understand the properties of the product fully and read the MSDS label.  There is usually a surprising amount of useful information available in an MSDS that is generally not shown in the brochure.  The application guide usually gives you only part of the information.  If the retailer can't supply the MSDS sheet, search the Internet.  And lastly, the same product is NOT necessarily best for all applications.  So don't stick to one manufacturer's goop!  (no pun intended). 

The terms sealant and adhesive are quite often interchanged, usually without a problem.  However there are subtle differences.


SEALANT - A sealant is designed to form a watertight and airtight seal between two pieces.  It can also be used to join two pieces (often in conjunction with a mechanical fastener to prevent movement), or to electrically isolate one piece of metal from another to prevent electrolysis (corrosion) or electrical noise (static).  Expect an application to last approximately 7 years.

Properties - A sealant cures to a tough, flexible, rubbery consistency but usually requires mechanical assistance for bonding.  A sealant remains permanently flexible without cracking or losing adhesion as it can generally withstand about 5% movement of the surfaces to which it is stuck.  A sealant is intended to keep a liquid in or out where there is no stress on the barrier.  (under a cleat, block, etc).  It skins over in about 10 minutes and cures to 80% strength in one hour, full strength in 24 hours.  No movement is allowed during the curing period.

  • Silicone Sealant - This is the sealant that most people are familiar with.  Your typical bath tub sealant falls into this category and there are oodles of manufacturers all claiming theirs is the best.  Many manufacturers produce a marine version of this product which doesn't smell like vinegar by the way.  If it does, take it back and tell the retailer you marine grade sealant.  When cured it is 5% elastic and highly resistant to chemicals.  It is excellent as an electrical insulating barrier between dissimilar metals to prevent electrolysis.  The adhesive strength is not as strong as polysulfide or polyurethane.  Compatible with plastics.  It MUST NOT be used to seal a through hull fitting, even if the fitting is above the water line.  Be aware that a form of bacteria may grow in the cured translucent sealant, which is why a version with a mildew inhibiter is available.  I've seen it growing on a lakeshore cottage window and it is evident in many shower stalls.  Eventually the water creeps behind it and releases the bond.  At this stage it is very easy to remove with a thin blade screw driver or razor knife. 

  • Amazing Marine Goop - Marine Goop is a tough sealant/adhesive that stands up to harsh weather.  It bonds whatever, forever with more tenacity than silicon sealant.  Permanently seals and sticks with a tight, flexible waterproof hold.  UV-resistant.  * For maximum UV resistance, paint over Marine GOOP. *  Dries clear, rubber-like consistency that's flexible, so it wont crack or break under stress. 
    This is a good choice to seal an instrument against the bulkhead.  It seals my magnetic compass which is removed for winter storage to protect the rubber membrane against drying.  I use it for quick repairs when underway on the water because it can be applied with very little mess and can stick to canvas, fabric, fibreglass, glass, nylon, most plastics, vinyl, waterproof cloth, rubber and wood very well.  It cures to touch dry in 10 minutes.  It is valuable to keep on board because it has such a wide range of applications.  It sticks 


ELASTIC ADHESIVE - An adhesive sticks to most things with enormous shear strength.  This is excellent for sealing a joint or sticking things together top prevent horizontal movement.  The elastic properties make it excellent to withstand movement, for example between a through hull fitting and the hull, or to keep a liquid in or out.  The adhesive has to stay stuck during the slight movement as the hull flexes around the rigid fitting or when a fitting is bumped, causing it to deflect.  Since the adhesive film may be only 1/32" thick, a 1/8" movement requires 200% elongation.  This is quite remarkable.  The adhesive only needs to adhere well enough to stay in contact with the fitting and the hull; the backing nut keeps the fitting in place, not the adhesive.

  • Properties - Similar to a sealant but does not require mechanical assistance for bonding.  A good elastic adhesive can withstand ~50% movement of the surfaces to which it is bonded so it is excellent where there is stress on the barrier. (Thru hull fitting, chain plate, toe rail, window, teak deck).  It usually skins over in ~30 minutes and cures to 100% strength in 24-48 hours.  No movement is allowed during the curing period.

Polysulfide Adhesive - One of the most versatile adhesives available.  Two-part polysulfide has long been popular as a sealing material for teak decks.  One-part polysulfide is easier to use and just as durable, but slower to cure.  It bonds well to most surfaces.  Oily woods (such as teak) should be primed before applying polysulfide.  It will dissolve plastic.

Polyurethane Adhesive - Recommended for permanent bonding because of its enormous adhesive strength.  Good for hull/deck joints and bonding a through-hull fitting.  It is incompatible with ABS and Lexan plastic.  Sikkens Sikaflex and 3M 5200 are examples of a high quality elastic polyurethane adhesive.  When cured the bond may be considered permanent but it is quite easy to separate with a razor blade or thin sharp knife.  Just cut through the cured bead and separate the two halves.  Then push the excess off with a sharp chisel or similar tool.  This leaves a perfect surface for the next application.  Cured 3M5200 is tough enough that in some case has a tendency to remove fibreglass with it. 
Another excellent polyurethane adhesive is Gorilla Glue which is ideal for joining dissimilar materials such as metal or plastic to wood.  It has an open time of 15 minutes, sands without clogging and absorbs stains well.  Squeeze out is easily removed leaving a water proof glue line.  It is ideal for making a wood cockpit grate.

Butyl Rubber - Butyl rubber is a very old product that comes in a spiral roll separated by waxed paper to preserve it, making it easy to apply.  You simply unroll the end of the bead and cut off how much you need.  Then you knead it to shape and apply to the area to be sealed by handling the paper, not the butyl rubber.  Before applying it, sanitize the surface with acetone.   Butyl rubber is used to seal the hull to deck joint of a San Juan sail boat, plus many other sailboats.  It sticks extremely well, without the potential application mess of Sikaflex or 3M5200, and stretches to well beyond 300% before the bond is broken.  I now use butyl rubber for bedding components to the deck because it is so easy to apply, sticks so well under load and is long lasting.  40 years and longer if done correctly.  I have seen it in black and grey.  Try your local automotive glass shop.  They should sell it or maybe you can trade the tail end of a roll for a coffee.  Here's a great link on how to apply butyl rubber.

Urethane - Urethane is typically used to bond and seal a North American windshield to a vehicle.  The bond is so good that the windshield forms a structural integrity of the vehicle.  Urethane comes in a caulking gun tube that makes it easy to apply.  It sticks extremely well to fibreglass, does not sag, and is excellent for filling an exterior gap or joint.   Before applying, clean and sanitize the surface with acetone.   It requires humidity to cure.   Once cured it remains flexible to withstand +/- 35% joint movement and can handle foot traffic.

Tremco Gutter Seal - This one part elastomeric sealant is formulated from a blend of polymers to produce a highly adhesive and flexible sealant for a rain gutter.  It is more of an adhesive than a sealant which cures to flexible rubbery set that is serviceable and tack free overnight.  Full service cure is achieved in two weeks at 25C0.  It has excellent resistance to UV, ozone and water, being specifically designed for the huge amount of thermal expansion that gutters experience.  Excellent immersed adhesion.  Marty P. Schimmel.

NOTE - "I have long searched for a sealant that can be applied and sticks underwater and this is the first one that does the job.  To test it I simply squeezed out a bead on a sheet of immersed gel coat, holding the end of the nozzle directly against the clean gel coat.  As it oozed out it stuck on the first pass.  While it can seal a hole underwater, it does not cure.  After 3 days under water my sample test remained as pliable as it was inside the tube.  However, once I pulled the sample out of the water, it cured overnight. 
- You have to be a bit judicious when applying this product because a pliable uncured sealant can only hold a minimal amount of water pressure till it gives.  So if you are thinking of using this product to plug a hole below the water line I suggest covering the sealant with a flexible or rigid plate.  This should help to prevent the water from pushing the sealant into the hole and ultimately leaking into the hull.  Sealing the backside of the centerboard support plate should work just fine if you are not long from your haul out." 


RIGID ADHESIVE - A rigid adhesive sticks to things with enormous shear and pull strength.  This is excellent for bonding similar and dissimilar materials, which is the primary purpose.  In general, epoxy is designed to withstand movement in a joint, for example between two components of a hull that must not flex.  It is the tenacious holding property of epoxy that makes it so excellent at maintaining a joint to keep the boat afloat.  It is also designed to saturate porous material to improve the strength and preserve it. 

EPOXY - Epoxy sticks to polyester better than polyester does to itself.  This is one of the best reasons why you should use epoxy to repair or renovate your boat.  Epoxy always comes in parts A & B, to be mixed with strict ratios.  There are many manufacturers of this wonderful goop.  Probably the best known epoxy is WEST made by the Gougeon Bros who manufacture WEST System Epoxy.  For low temperature repairs I like Fiber-Tek Cold Cure epoxy.  However, it may not be  convenient to mix a small batch for on the water repair.  JB-Weld makes many excellent epoxies that come in small tubes.  The one that holds best under water is JB-WaterWeld stick.  5 Minute Epoxy comes in small tubes, part A & B, mixed in equal amounts dragged out on a sheet of paper for mixing.  Clean up with soap and water before it sets. 
 NOTE: Do not use epoxy with chopped strand mat or fibreglass mat.  The binder that hold the loose strands of mat together cannot be dissolved by epoxy resulting in poor absorption of epoxy into the mat.  Fibreglass mat work very well with polyester resin.

LINKS - Go to Sikkens Sikaflex marine adhesives and sealants for technical specifications.  Cured Sikaflex can compress or expand up to 50% without detaching from the surface.  Sikaflex is available in select chandlers, some glazing shops and home renovation stores.

HOSE FITTINGS - To transport a fluid.

  • If the fitting is not to be taken apart, use an adhesive sealant.

  • For a 900 or 1200 fitting with a hose connected the recommendation is to use an adhesive sealant as it helps prevent the fitting from turning from the weight of the hose.

  • If a fitting is to be taken apart with no risk of turning, thread tape or a lesser strength sealant such as 3M4200 can be used.  A backing nut can be used to prevent turning.

  • When tightening a threaded fitting, do not over tighten, simply let the sealant provide the seal and orientation of the fitting.

  • If tightening is required, tighten to a maximum of 16 Nm (12 ft/lb).

  • Never screw a straight thread into a tapered one.  The contact area is way too small.

ADHESIVE THREAD SEALERS - (Always check manufactures product literature before use)

  • SIKAFLEX® 291i Marine Adhesive Sealant
    A one-part polyurethane adhesive/sealant. Starts to cure (tack-free) in approx. 2 hours, after which hoses can be attached. Full cure takes 24 hours refer to product literature. Creates a permanent seal. Available colours = Black, White.

  • SIKAFLEX® 591 Multifunctional Sealant for Marine applications.
    A multipurpose sealant designed for marine applications. It is suitable for elastic, vibration-resistant joint seals.

  • 3M™ Marine Adhesive Sealant Fast Cure 5200
    A one-part polyurethane adhesive/sealant. Starts to cure (tack-free) in approx. 2 hours, after which hoses can be attached. Full cure takes 24 hours refer to product literature. Creates a permanent seal. Colour = White.

  • Bostik® 920 Marine Sealant
    A one-part urethane adhesive/sealant. Starts to cure (tack-free) in approx. 2 hours, after which hoses can be attached.  Full cure takes 1.5 3 days refer to manufacturer’s product literature


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