Replacing the Sealant on your Boat


man applying sealant to boat hull

It may seem to be one of these quick, easy jobs you do before you take your boat out this summer but removing and replacing old sealant can be anything but simple. The old sealant may not want to budge and then there’s the decision of which new sealant to use.

What Types of Sealants are Available?

There are a few things to consider when choosing the right sealant for your boat. You want to choose the right compound to suit the materials you are bonding and their position in relation to your boat’s waterline. For a long-lasting bond, use the most appropriate sealant.  


For a high-performance polyurethane above or below the water line, use 3M 5200 Adhesive Sealant to seal wood to fibreglass, hardware, hull and stern joints. The permanent bond remains strong, flexible, cures within 5-7 days and is tack-free in 24-48 hours. The product won’t sag or flow into seams and you will have peace of mind knowing it is resistant to weathering and salt water.

Used to bond most materials, the Sikaflex 291 is elastic and resistant to weather and ageing. Like the 5200 it can also be used above or below the waterline and will bond to wood, metals, metal primers, paint coatings and ceramic materials. This sealant can not be used on Perspex or polycarbonate as these are prone to stress cracking.


If you need a flexible, above the waterline, moisture-curing silicone, 3M’s Marine Silicone Sealant Mildew Resistant could be the one. It can be used on painted metal, fibreglass, glass, non-oily wood, abraded rubber and many plastics. This silicone will remain flexible and is resistant to saltwater and high temperatures. Suitable for interior and exterior sealing, it won’t yellow, shrink or sag with age.

The Sikasil Marine Adhesive Sealant is another silicone sealant suitable for sealing windows, hatches, fittings and instruments, as well as for bonding small components which are not subject to high loads. While this sealant cannot be used below the waterline, it provides a flexible bond resistant to thermal, vibration, UV and weathering. This type of sealant is ideal for applications where the parts may be required to be removed at a later time.

Silicone/Polyurethane (also known as elastomeric)

A multi-use above and below the waterline sealant, the Sudbury Marine Sealant can be used in the place of polyurethanes, polysulfides and silicone sealants. It is surface resistant to salt water, teak cleaners, oil, petrol, diesel fuel. Its versatility includes being able to twist, bend, expand and compress. Available in several colours, this sealant can also be sanded and painted if required. Most sealants have a shelf life of 12 months after opening but with this one, you can use it for roughly three years.


We stock all types of Boat Maintenance products to keep your boat afloat.

Browse our Range of Maintenance Products or Call us on 1300 308 161


Removing Old Sealant

Many fittings won’t want to come off easily and removing a stubborn one can be a long and frustrating job. Some old bedding compound sealants (ie silicone, acrylic caulk, butyl rubber, polysul-de or even polyether sealant) will give way without too much fight if you pry to break the bond or stretch it beyond its capacity.

However, if the sealant is polyurethane such as the popular 3M 5200 it’s a different story. You probably won’t be able to pry it apart without damaging the fitting or boat.

Follow these steps combined with some patience and you should be able to (eventually) free all types of fittings.

  • For mechanical fasteners, unscrew bolts with a screwdriver, possibly one with a wrench on the shank and then use a manual impact screwdriver, if needed.

  • Using continuous pressure and elastic sealants will eventually elongate or release the sealant. Using a hammering approach isn’t the best as you can destroy the fitting or if it does come away, it may take some of the boat’s gelcoat with it.  If pressure doesn’t work, try cutting the bond. Use a box cutter or carpet knife to slice the perimetre and break the bond. If the knife doesn’t cut it, use a wire saw. A 30cm length of thin wire (i.e. braided fishing line or guitar string) with two loops at the either end will allow you to use a sawing motion between the bonded parts. Try a wedge to pry behind the wire. If you still don’t have any luck with the wire, try a heated knife to soften the cured polyester. Just beware you risk damaging the cured polyester with the heated blade.

  • If it’s a stubborn plastic fitting, score the edge of the old 3M 5200 and wedge it so that you have exposed fresh sealant to spray. Repeat applications until you have broken the bond.

Applying New Sealant

It can be messy and frustrating but with these tips, you can achieve a good-looking and long life seal.

  • Primer – use a primer such as Penetrol Primer to creep into crevices and tiny holes to provide a powerful bonding film. A primer can improve adhesion rates of your sealant by around 20%.

  • Beading – a triangular bead of sealant will give you the best result for bonding and help reduce wastage. Cut a notch on one side of the nozzle and apply with the gun perpendicular to the surface. Keep the nozzle a set distance from the edge of the material.

  • Sealing Deck Fittings – if you are using sealant on deck fittings, screw or bolt them down to almost tight before applying the sealant. Once the sealant has cured, give the fitting a final tighten to compress the sealant.

  • Curing – Polyurethane sealants rely on moisture to cure, and if you are bonding materials with little moisture in them such as fibreglass or steel, you may need to apply a very fine mist of water to help kick-start the curing process.

  • Caulking – if you are caulking between wooden planks or seams of teak, you can either overfill the seams then cut away the excess with a sharp chisel. The other option is to mask off each joint to cover surrounding teak then apply the sealant and smooth with a scraper before it cures being careful not to leave a hollow. The masking tape needs to be carefully peeled away before the sealant finishes curing too.

  • Applying Bedding Compound – snip off the end of the Sikaflex 290 sausage-like then squeeze the sealant onto the deck and spread it out with a notched spreader for an even layer.   

Keeping Sealants Fresh

Most sealants will start going off as soon as they are opened. If you may want to keep your sealant nearby for quick sealing jobs you see, store it in a waterproof container on your boat. Just remember to make sure you are using the sealant that matches the materials it’s sealing.

Moisture-cured sealants will go off quicker in hot, humid conditions, so it is best to store in a fridge. Keep the nozzle attached to all sealants for a longer life.

For the best sealing job, follow the product instructions and don’t hesitate to call Boat Accessories Australia on 1300 308 161 or send us an email if you have any queries.   

Category: Maintenance

Related Articles

How to Prepare Your Boat for Winter Storage

How to Prepare Your Boat for Winter Storage

Thankfully, our Australian winters are not as extreme as our fellow boaters’ overseas who often face sub zero temperatures. Nevertheless, there are steps that need to be taken to ensure you’ll be ready to hit the water again once spring arrives.