Understanding marine sealants & adhesives

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: