Are you wondering what marine sealants are suitable to use on your boat? As a liveaboard, I once woke up to rain pouring onto my face from a leaky hatch and it was then that I realised the importance of a quality marine sealant. Before we can begin our journey into picking the right sealant, it's important to know the difference between a sealant and adhesive sealant.

Knowing The Difference

An adhesive sealant can be used to seal fittings but also has adhesive properties so it can mechanically bond to certain materials. In short, you could stick your friends bum to a chair using it, but the prank may take up to 24 hours as it can take a long time to cure.

A non-adhesive sealant will generally rely on the aid of a fitting, screw or bolt to create a waterproof seal. With a normal sealant, you are essentially creating a waterproof gasket. You can't rely on normal sealant to hold anything in place; you are relying on a method of fastening, for example; a screw or bolt.

Types Of Sealant

Silicone

Butyl

Hybrids (using the best elements to create better products)

Polysulphides

Polyurethane

Silicone



Hybrid



It is said that silicone is an inert synthetic polymer compound and is technically part of the rubber family. You can imagine why its been a favourite in boat maintenance for some time. Its UV resistant, heat resistant, a great insulator and is ideal for creating gaskets. Its downfall is that it makes for a weak adhesive and for this reason sometimes is unpractical for use below the water line.

Hybrids are combinations of the best elements of existent sealers. The idea being that each type of sealant has it's pros and cons so they just pick the pros from the best types. For example the strength of polyurethane with the UV heat resistance of silicone. They will bond better but aren't as strong as polyurethane.

Polyurethane

Polyurethane sealants create an extremely strong mechanical bond when used on the correct surface. They are UV resistant and can be used below the waterline. In the aviation industry on unpressurized aircrafts, windows are sometimes attached using adhesive sealant only. This is a testament to the strength of modern sealants. It's notable that polyurethane is not compatible with acrylic perspex and other types plastic glazing.

Polysulfides

Polysulfides; a synthetic rubber, forms a strong mechanical bond and has an excellent resistance to UV, fuel and oil as well as good general corrosion protection. Polysulfide also makes a great electrical insulator and is resistant to vibration, shock, impact, and thermal changes. It can be easily painted.

Butyl

Butyl is a non-adhesive sealant normally found in the form of tape and has the characteristics of chewing gum, but can now be purchased as sealant. It's great for serviceability and it's easy to remove and re-apply as well as being a lot less messy than traditional sealants. It could last 20 years but it won't look great. It's susceptible to UV damage and can often look like a bit of old chewing gum. It's not suitable for use below the waterline.

We have put together a list of marine sealants followed by some important information to read prior to picking one.

Popular Marine Sealants

Bostik Simson ISR 70-03 - £10 inc shipping - Sealant And Mild Adhesive

Tensile Strength - 2.6 MPa.

Elongation Break - 250%

Strong UV resistance and great ageing properties.

Solvent, isocyanate and PVC free.

Resistance to fresh and salt water making it ideal for use below the waterline.

Fast curing.

Flexible once set and permanently elasticated within temperatures from –40°C till +120°C.

Can be painted wet on wet.

Compatible with the majority of industrial paint coatings.

Can be sanded after curing.

Available in white, grey, black, dark grey, brown, yellow.

Arbomast BR - £8.00 - Sealant

ISR 70-03 is a high-grade, one component, silyl modified polymer marine grade sealant that has been specially formulated for the rough and tumble of industrial environments. It's fast, strong and elastic properties make it ideal for sealing deck equipment. This sealant also adheres to the majority of surfaces used in boat building including aluminium, stainless steel, brass, powder coated surfaces, galvanised steel, copper, most lacquered surfaces, glass, brass, powder coated surfaces, zinc, polyester and grp and most impressively Lacquered wood.

Arbomast BR is a one component Butyl rubber based sealant that

Tensile Strength -Unknown.

Elongation Break - Unknown.

20+ year service life.

Easy applications.

Available in grey, teak and black.

Geocel Marine Rubber Silicone - £13 inc shp - Sealant

can be used for bedding deck hardware. It has mild adhesive properties. Sources have claimed it has a proven service life of 20+ years. Like other sealants, it will adhere to most construction materials and substrates. The downside is that it has low UV resistance meaning it isn't suitable for areas exposed to sunlight.

Geocel is a one component flexible silicone sealant that has been specifically formulated for a watertight seal which is resistant to the marine environment. It can also mildly adhere to GRP, most metals, glass, painted or varnished wood, and most plastics. It is particularly good for sealing around hatches and port holes, locking nuts and bolts against vibration, the insulating of electrical connectors, sealing plumbing pipework, creating in-situ gaskets and creating watertight joints throughout.

Tensile Strength -2.3 MPa.

Elongation Break - Unknown.

High flexibility.

Resistance to salt and fresh water.

Resistant to high vibrations.

Available in white, black, clear.

Sikaflex 291i - £13 inc shp - Adhesive Sealant

Tensile Strength -1.8 MPa.

Elongation Break - 500%

Vibration-resistant.

Resistant to fresh water, seawater, limewater, sewage effluent, diluted acids and caustic solutions; temporarily resistant to fuels, mineral oils, vegetable and animal fats and oils.

Not resistant to organic acids, alcohol, concentrated mineral acids and caustic solutions or solvents.

Available in white, black, brown.

Puraflex 40 - £8.54 inc shp - Adhesive Sealant

Sikaflex 291i excels in its mechanical bond strength and is a polyurethane sealant. It will bond with most of the common building materials used in boat construction and maintenance. It will bond to wood, most metals, metal primers and paint coatings (2-C systems), ceramic materials and GRP. However, it isn't suitable for use with plastics that are prone to cracking, for example, acrylic boat windows.

Tensile Strength -1.5 MPa.

Elongation Break - 700%

Excellent chemical resistance.

High mechanical bonding strength.

Resistance to salt water.

Available in white, black and grey.

Aquaseal mSEAL 295 - £4.00 inc shp - Mild Adhesive Sealant

Although Puraflex 40 does not claim to be specifically designed for marine use, like Sikaflex its a one component polyethene with excellent mechanical resistance and bonding. Some resellers websites have claimed its use as a marine sealant as well as being backed by user testimonials on various boating forums. It can be used on concrete, wood, metal, aluminium, polyester, glass, uPVC, stone, and ceramics.

Tensile Strength -1.5 MPa.

Elongation Break - 600%

Strong UV resistance.

Silicone free.

Resistant to salt water.

Resistant to high vibrations.

Outstanding primer adhesion to most marine substrates.

Excellent adhesion to metals.

Available in white and black.

Evo-Stik Sticks Like Sh*t - £5.64 inc shp - Adhesive Sealant

mSEAL 295 is one component polyurethane adhesive and sealant that prides itself on its permanent elasticated and vibration resistant properties. It will bond to most marine substrates but is however limited to use for light stress loaded bonding. Notably, it also has a slower curing time and can not be used on surfaces that bleed oils or plasticisers.

Tensile Strength -1.8 MPa.

Elongation Break - Unknown.

Stays elasticated once formed.

High mechanical bond strength.

Resistant to water as well as oil & petrol.

Available in warm white only.

UniBond FlexTec FT101 - £12 inc shp - Adhesive Sealant

Evo-Stick Sticks Like Sh*it is a high strength single component silyl modified polymer (SMP) based adhesive and can most impressively be applied to damp and even wet surfaces. This makes it a great adhesive sealant to have on board for emergency use. It can be used on a wide range of materials including wood, concrete, glass, brick, ceramics, metals, rigid PVC, plaster, grp insulation boards and foam polystyrene. Notably, it won't bond to polyethene, polypropylene or Teflon.

Tensile Strength - 1.0 MPa.

Elongation Break - 400%

Easy to work with.

Strong UV resistance.

Available in white only.

3M 4200FC - £15 inc shp - Adhesive Sealant

UniBond FlexTec FT101 is a one component hybrid polymer which can be used to seal, fill and bond. It claims to be able to bond without the use of a primer to a wide range of materials but doesn't specify which ones it does not. It can also be applied on both damp and wet surfaces but does take 24hours to set and may not ideal for fast same day repairs.

3M 4200FC is all around one part polyurethane adhesive sealant that can be used to bond wood, gel coat and fibreglass. It can also be used above and below the waterline.

Tensile Strength - 1.24 MPa.

Elongation Break - 400%

Fast Tack-Free time - 1-2 hours.

Permanently elastic.

High vibration and shock resistance.

Available in white.

Everbuild Stix‐All - £10 inc shp - Adhesive Sealant

Tensile Strength -1.5 MPa. Elongation Break - Uknown. High bonding strength. Resistant to water, petrol, 10% dilute acids/alkalis and most solvents, Quick Curing (forms a skin in 10-30 minuets depending on temperature) Resistant to temperature extremes ‐40°C to +150°C



Available in black, brown, crystal clear, grey and white.

CT1 - £10.50 inc shp - Adhesive Sealant

Everbuild Stix-All is a one component adhesive sealant utilising the best elements of silicone and polyurethane technologies to create a hybrid. It hasn't been designed for marine use specifically but It is compatible for adhering with most the common materials and can be applied on both damp and wet surfaces. It will bond to metals, plastics, glass, concrete, plasterboard, plaster, polyester, perspex, glass, wood, enamel, painted surfaces and GRP.

Tensile Strength -1.72 MPa.

Elongation Break - 350%

Resistant to seawater, aliphatic solvents, oils , greases and diluted organic acids.

Available in clear, white, black, grey, beige, brown, oak, blue and silver.

3M 5200 - £24 inc shp - Adhesive Sealant

A favourite amongst boating enthusiasts CT1 is suitable as a universal adhesive and will work in all applications. CT1 has no shrinkage and can be applied in both damp and wet conditions. It adheres to most metals (including lead) as well as glass, mirrors, wood, polystyrene and GRP.

Tensile Strength -1.72 MPa.

Elongation Break - 874%

Great flexibility once set.

Resistant vibration, swelling or shrinking.

Available in white.

Sabatack 720 - £13 inc shp - Adhesive Sealant

3M 5200 is a one part polyurethane adhesive sealant which reacts chemically with water to deliver an extremely strong mechanical bond. Multiple sources including boat yard workers have claimed it's so strong its use as a marine sealant should be limited. It is apparently too strong for bedding equipment that may need replacing or maintaining in the future. However, It claims it is ideal for seals and joints both above and below the waterline. Debond by Marine Formula can be used to remove the sealant but be warned, it's expensive stuff!

Sabatack 720 is a one component MS polymer-based adhesive sealant ideal for use in the marine environment. It's primary meant to be used for sealing, seams overlaps and joints with bonding being its secondary function.

Tensile Strength -2.2 MPa.

Elongation Break - 300%

Fast curing.

Resistant to UV.

Resistant to high temperatures.

Avaialble in white, grey and black.

How to tell when its time for re-bedding?

Once you own a boat you are trapped in an eternal circle of labour, and whether it's for leisure, living or work purposes you will always need to maintain it. You can limit your workload by using the right method with the right gear. Sealing and re-bedding deck equipment most definitely fall's into the laborious circle but with the right adhesive or sealant and a bit of skill, you can seal your deck with no problems.

Your boat for obvious reasons needs to be tighter than a duck's arse; most importantly below the waterline, but don't disregard your topside leaks. They have the potential given adequate time and rainfall to sink a boat.

Things to look out for -

Dirt trails - your sealant has failed and is now allowing dirt to collect under your deck equipment. If it's not rust or corrosion then it may be that failing sealant is the cause. If your outer layer has perished then there is a chance that your seal will break next, so it's time to take action.

Water - visibly dripping through into your cabin during rainy days. Leaking water can often be seen forming small droplets on the bottom of protruding bolt heads. Wait for a forecast of rain or cheat and use a hose to find leaks. If you get your bilge bone dry beforehand, you'll be able to follow the trails of water from any leaky hardware by looking at your bilge areas with a hand torch. This method works amazingly well. If you have a dirty bilge it seems to be even easier and the trail seems to show up clearer where the dirt is wet.

Choose The Right Sealant

Lots of questions will need to be answered before wasting your money on the wrong kind of marine sealant. Each sealant bonds and reacts with materials in a multitude of ways. Some sealants, for example, will crack perspex boat windows if they are not made from an acrylic compatible chemical. There is nothing more frustrating than bolting and re-bedding all of your boat's windows, only for someone to come along at the end and tell you that you've used the wrong sealant - Doh!

Sealants were originally designed to patch holes in planes during the war but since then they've got a lot more complicated. Corporate competition means companies are continuously trying to outdo each other. The marine industry is constantly screaming out for products that last longer and perform better and subsequently, we are fed increasingly better sealants. A lot of companies who create sealants eventually wise up to the fact that their product will work in the 'marine' industry, and often subtly add it to their datasheet without it being a specific marine product and liable for leisure tax.

Don't worry breaking your requirements into some simple steps really helps.

Strength - how strong is the mechanical bond strength (some manufacturers give this information)?

how strong is the mechanical bond strength (some manufacturers give this information)? Flexibility - can the sealant move without breaking apart and can it absorb impact?

can the sealant move without breaking apart and can it absorb impact? Compatibility - will it work with your material you wish to bond it too? Will it be safe for below the waterline?

will it work with your material you wish to bond it too? Will it be safe for below the waterline? Serviceability - how easily will I be able to remove and replace the sealant, how long does it take to set?

how easily will I be able to remove and replace the sealant, how long does it take to set? Service life - how long does the manufacturer claim their product lasts and is it UV resistant?

We've even created a beautiful acronym for you to remember: 'SFCSS' (pronounced; Suu-Fuu-Cissss). See, if that's not catchy enough for you then I don't know what is!

Terms Explained

Tensile Strength - the resistance of a material to breaking under tension. To put it even clearer this is the amount of force a material can take until it breaks apart. This is measured in N/mm2 to MPa and can normally be found on a product datasheet.

Elongation Break - Is the % of stretching that the product breaks at. Some sealants have been designed to stretch more than others.

So to summarise, imagine you're an evil man wanting to pull someones' arms off using horses. Tensile strength is the amount of horse's you would need to pull them off which is measured in N/mm2 or MPa. The elongation break is how long the poor persons' arms will stretch before they rip off - ouch!

Thank you for reading! To help keep this website going, please share this article. Any experiences people have had with any of the above products, thoughts or ideas, please feel free to post below!

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