Marine Sealants – All You Need To Know

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


Geocel Marine Silicone SealantIt 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.


Sikaflex 29l

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; 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.


Arbomast BR Butyl Sealant 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

Bostik Simson ISR 70-03ISR 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.

  • 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

Arbomast BR is a one component Butyl rubber based sealant that Arbomast BRcan 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.

  • 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

GeGeocel 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 

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 -1.8 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 

Puraflex 40Although 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 – 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

Aquaseal mSEAL 295mSEAL 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 -1.5 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

Evo-Stik Sticks Like Sh*t 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 -1.8 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 

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.

  • 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 

3M 4200FC - Adhesive Sealant

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 

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 -1.5 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 

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 – 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 

3M 52003M  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!

  • Tensile Strength -1.72 MPa.
  • Elongation Break – 874%
  • Great flexibility once set.
  • Resistant vibration, swelling or shrinking.
  • Available in white.

Sabatack 720Sabatack 720 – £13 inc shp – Adhesive Sealant

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)?
  • Flexibility – 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?
  • Serviceability – 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!

18 thoughts on “Marine Sealants – All You Need To Know”

  1. Great article, its taken out the complicated selection of what to use and where and when to use it. Thanks for taking the time to simplify the sealant mystery.

    1. Great article.
      I searched through it and did not find a sealant that is good for sealing acrylic glazing on boats.
      Any ideas?


      1. Hi John , I have heard really good things about Arbomast br which is a butyl sealant, it doesn’t have any adhesive properties but by the sounds of it and the feedback I have heard from others it will seal fittings and windows etc for decades without problems. And if you do ever need to re-apply it’s easy to remove and start again.

  2. Hi Louis, I’ve just read your sealer article, but I’m still unsure which one I should use for sealing around my perspex windows and psinted aluminium window frames.
    It requires good adhesion to both surfaces and a degree of flexibility to allow for expansion / contraction in hot and cold weather.
    Which one woild you recommend?
    Cheers Rich

    1. Hi Rich, If you are using aluminium frames then essentially the sealant will only need to be used to create a gasket. There is my opinion is no need to have a good adhesion if the window is held tight by rivets or bolts and a frame. Aquaseal mSEAL has been known to work well for this purpose and Geocel Marine Rubber Silicone is something I have personally used with good results.

      Here is the link to the datasheet for Aquaseal –

      Here is the link to the datasheet for Geocel –

      The only catch 22 that silicone can also affect aluminium so there are pros and cons to both.

      Arbomast BR is also a butyl based sealant that will last for 20 odd years but won’t give you any adhesive properties. I have only tried Geocel so can only recommend that one but I have heard amazing things about Butyl based sealants for perspex.

  3. I want to build a 16′ length X 48″ Beam jon boat out of 1/2″ plywood to use in small protected lakes, powered by 10 hp outboard. Unfortunately I weigh 385 lbs and want to know if 1/2″ marine plywood will be strong enough for me to walk around boat while fishing? I realize the transom will have to be thicker or stronger, to handle extra weight of outboard and force generated wile pushing boat . Which adhesive sealant would be best above and below water line? Most plywood comes in 8 foot lengths so I must bond 2 sheets end to end for bottom can I use same adhesive sealant to glue and seal joint where sheets meet? Please provide recommendations,

  4. I have slight leaking through the deck to hull joint on my 37 year old moody29. I can see the joint and it looks as though the original sealent has gone a bit porus. There is a grouve so I am thinking that I could over fill with a sealent then smooth it down. Do you have a sealent you could recommend and supply. Hull and deck are glass fibre with an aluminium toe rail.

  5. I wonder can any of these bond to a Osmo oil used to finish a kitchen counter? A friend recommended 3M Marine Adhesive/sealant but I am not sure it will stick to the wax/oil…will it?

    1. Hi Alex thanks for your question – if we are talking about a table top that has recently been oiled I would personally lightly sand the area that you want to adhere with some 220 grit. Most adeshive sealants will struggle to stick to an oiled surface. Contradictory to this I recently used an old oak table that had been waxed for a worktop. I had to seal and mount a stainless steel sink with bathroom sealant. I forgot about the wax but despite of that it’s still holding strong. Long story short; most things will work it just depends how long you want them to last. Aslong it doesnt compromise safety or structure what have you got to loose. Hope that helps.

  6. For acrylic windows, Dow 795 in a thick layer works well. A 30″ foot wide window needs a quarter inch , because acrylic expands and contracts with temperature. There are some Sikaflex and Boatlife products that work as well – check their specs. Thanks for posting the elongation break.

  7. We’re resealing windows on a 1989 47’ Buddy Davis. It’s fiberglass to glass.. old material is removed and area is very clean.. what type sealant do you recommend?
    Thank you.

    1. Hello Lee- thanks for getting in touch are you able to explain a little more. When you say you’re resealing – are you trying to bond glass directly to fibreglass with no other mechanical fixtured aka screws or bolts?

  8. Hi,

    Read your review and found it very interesting.

    But I’m stuck.

    I tried a butyl sealer on my Perspex windows. It just peeled off when I tested them.

    I’m unsure of what to use to bond the windows in place. I wanted a very clean finish so avoided bolts and such.

    Any advice?

    1. Hello Steve – Most Butyl sealants don’t offer any adehsive properties especially Arbomast BR. Arbomast would work fine if you are using other mechanical fittings such as screws, bolts or rivets. With Butyl sealants you are essentially creating a non-setting gasket. If you want to bond the perspex to the fibreglass you must use an adehsive sealant that doesn’t affect or damage perspex. If you look at the datasheet for most adehsive sealants they will provide you with a list of suitable applications.

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