Select Page

Sealants, adhesives and adhesive sealants

There is a bewildering variety of sealants, adhesives and even adhesive sealants available for marine use. When making a choice it helps to understand some fundamentals about all the chemical wizardry that has created this vast range of products, often misunderstood by boat owners.

While your local chandlery or boatyard manager might be very helpful and knowledgable and happy to help you choose the best product for a specific job, learning the basics of this yourself will make life a lot easier. It should avoid that very unpleasant feeling when you discover you have bought and used a product that was not the right one for the job. So what are the basic differences between a sealant, an adhesive and an adhesive sealant?

Here are some fundamentals to consider:

  • A sealant essentially does a similar job as a washer or gasket. Its function is to provide an airtight or watertight seal. Sealants are not adhesive and require fittings to be fastened with screws or bolts to create watertight seals.
  • An adhesive is a glue which binds two materials together.
  • An adhesive sealant is a sealant with adhesive qualities added.
  • Only marine grade sealants should be used on a boat. This is partly because of the curing agents they use, partly because they are designed to withstand the marine environment and partly because you want them to be waterproof.

Choosing the right sealant

When it comes to choosing a marine sealant, you need to be clear about the purpose you are using it for, then consider the options out there and work out the best sealant for the specific task you are doing. For example, is it for sealing a deck fitting, is it for use below the waterline, is it going to be exposed to plastics or solvents, do you need a permanent seal or one that will be removable? Other things to consider are how long is the cure time, how long will it last and whether you can paint over it.

Types of sealants are sub-divided into polysulphides, polyurethanes, silicones, polyethers (also known as hybrids) and butyl rubbers:

 Polysulphide

Polysulphide is a synthetic rubber with good adhesive characteristics. It is a good bedding compound, grips well to surfaces and allows for some movement caused by stresses and changes in temperature. It is used for caulking teak decks and bedding teak rails. It is resistant to UV, oil and fuel. It also makes a good electrical insulator and can be painted.

Polysulphide should not be used on plastic surfaces (especially acrylics such as Plexiglass and Perspex) as it causes many types of plastic to harden and split.

Polyurethane

Polyurethane is a very strong adhesive and creates a permanent bond. It can be used below the waterline and is UV resistant. It is used for skin fittings, GRP hull to deck joints, bedding keels to hulls and bedding toerails.   

Polyurethane should not be used on most types of plastic – therefore like polysulphide is no good for sealing plastic windows and perspex. It should not be used where sealing is the main objective, rather than permanent bonding, for example on items which you may want to take apart in the future.

Also, polyurethane is affected by cleaning solvents, teak oil and diesel oil so is not suitable for bedding deck hardware.

Silicone

Silicone is ideal for creating gaskets – it is best to think of it as a gasket material. It is stretchy and compatible with plastics. Silicone is used for bedding windows and is a good insulation between metals, so is particularly useful when fixing stainless steel fittings to aluminium masts, reducing the risk of galvanic corrosion. It is heat and UV resistant, non-shrinking and is a good insulator.

Silicone should not be used below the waterline and is a very weak adhesive. Marine grade silicones use alcohol or water cure agents. Silicone sealants that use acidic cure agents, which have a distinctive vinegary smell, are not suitable for marine use. These are widely used for domestic use. Silicone sealant cannot be painted over as it resists paint.

Polyether

Polyethers are better known as hybrid adhesive sealants – that is part silicone, part polyurethane or sometimes part polysulphide. The aim is to combine the flexibility and elasticity of a sealant with the strength of an adhesive, resulting in a durable product that seals, bonds and fills. Their list of properties sound almost too good to be true – claiming to be fast-curing, low odour, high adhesion, non-sagging, non-corrosive, non-yellowing. They provide durable, permanent watertight seals for joints subject to structural movement. They adhere to metal, glass and wood and are mildew resistant and acid free. Added to this impressive list of attributes most can be used above and below the waterline. And they are environmentally friendly.

Butyl

Butyl is a gasket type sealant with virtually no adhesive properties that makes a good alternative to silicone sealant. It is a synthetic rubber, impermeable to air and its first major application was for tyre inner tubes. For marine purposes butyl is usually manufactured in tape form with paper backing. It is malleable, does not harden and is considerably less messy than silicone to use. It is useful for small jobs as you simply cut off a bit of tape from the roll and use what you need – you can even put any excess back with the tape and keep it for future use. Butyl works well as a bedding compound, is ideal for bedding deck hardware and sealing windows. It never hardens but stays soft indefinitely.

Butyl should not be used below the waterline. It has low resistance to chemical solvents, particularly petroleum.

In summary, consider the following when choosing a sealant:

  • Bonding versus sealing.
  • Flexibility and stretch.
  • Suitability for above or below the waterline.
  • Compatibility with other materials.
  • Resistance to ultraviolet, weathering and chemicals.
  • How long it will last.

And finally, remember that sealants and adhesives get everywhere. Wear overalls or old clothes and disposable gloves to protect your hands.

A simple guide to understanding tides when passage planning

Understanding tides when passage planning When planning a trip in tidal waters, check the tides before going afloat. Use...

Boat Handling – anchoring

Anchoring your yacht or motorboat Anchoring is one of the most important boat handling skills. If you can set an anchor...

Essential Knots: Bowline

Essential Knots: Bowline Use: Making a secure eye or loop in the end of a rope. Bowlines have many uses on a boat, for example to make a...

Distress flares – which flare, how & when to use?

How to use distress flares at sea Flares should be kept in a waterproof container in an easily accessible location such as a...

Top 5 Reasons Why an Inflatable SUP Should Be Your Next Yacht Accessory

In this article, inflatable paddle board expert Jason Paul gives the top 5 reasons why an inflatable SUP should be your next...

Boat surveys

A full boat survey assesses the condition of the hull, mechanical gear and means of propulsion. The survey is carried out with the boat...

Galvanic and electrolytic corrosion

Galvanic corrosion is an electrochemical reaction between two or more different metals, in the presence of an electrolyte (note salt water is a good electrolyte).

Essential Knots: Clove hitch

Essential Knots: Clove hitch Use: Tying a rope to posts, bollards, rings or a guardrail. Step 1. Make a turn around the object and lay...

Understanding marine sealants & adhesives

Sealants, adhesives and adhesive sealantsThere is a bewildering variety of sealants, adhesives and even adhesive sealants available for...

Essential boat engine checklist

Boat engine checklist Engine oil level check Even if you have checked it previously, confirming the engine oil level is up...

Wooden Hulls – Part 1

Traditional wooden boats have a plank on frame construction, a centuries old boat building method that is still in use today. Variations of the traditional method include carvel, clinker and strip planking, which all relate to the way the planking is attached to the frame.

You Need To Understand The IRPCS ColRegs To Pass Your Yachtmaster, Master of Yachts and Coxswain Certificate of Competence

IRPCS ColRegs Rules of the Road at Sea and Yachtmaster Learning, understanding and remembering the International Regulations...

Gybing a sailing boat

Gybing is the sailing manoeuvre used to change a boat's direction through a following wind. As with the tacking manoeuvre,...

ColRegs Nav Lights & Shapes, Rules Of The Road and IALA Buoys Apps

ColRegs Nav Lights & Shapes, Rules Of The Road and IALA Buoys Apps Make Learning Rules on iPhone, iPad, iPod and Android...

Steel hull maintenance

A steel boat owner’s biggest enemy is corrosion. You don’t have to worry about osmosis or rotting timbers, instead rust is the number one issue that will keep you awake at night.

Essential yacht tender safety for skippers and crew

Essential yacht tender safety - the dangers inherent in using a dinghy to get ashore from a moored or anchored yacht are all too easily...

Keel design – options to consider when choosing a yacht

Keel design is constantly evolving and nowhere is this more apparent than in modern racing yachts such as the Imoca Open 60...

Passage planning and pilotage

Passage planning and pilotage help skippers navigate safely from one port to another. A passage plan takes into account all...

Keel maintenance and Repair – Part 2

If you have ever witnessed a boat colliding with a rock or other submerged obstacle you will know that there is an almighty thump and the whole boat shakes and judders. While such hard groundings seldom result in catastrophic keel failure, something has to give and even the sturdiest keels can easily be damaged by such an impact.

Boat decks and superstructure

The deck of a boat is constantly exposed to the elements and should be inspected on an annual basis. Particular attention needs to be given to the overall condition of deck fittings such as the stanchions, cleats and chainplates.

The give-way hierarchy at sea – who gives way to whom?

Whatever their size or type, all skippers have a responsibility to avoid collisions with other boats at sea.  It is...

First Aid Afloat – fish spine injury

First Aid Afloat - Here is what to do if somebody stands on a fish spine: • Check for dangers. Is it safe for you to enter...

ColRegs Rule 14 – Head-on Situation

  ColRegs Rule 14: Head-on Situation (a) When two power-driven vessels are meeting on reciprocal or nearly reciprocal...

Understanding boat engines

Irrespective of what kind of engine a boat is equipped with and who does the work, the regular care and maintenance of a marine engine is essential. The most common cause of marine engine failure is widely known to be lack of maintenance.

Steel and Aluminium Hulls

The two metals used for hull construction are steel and aluminium. These are both very strong materials and will last a long time as long as they are cared for, which primarily means protecting steel boats from rust and aluminium boats from electrolytic action.