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Author: Boat Accessories Australia Date Posted: 10 January 2018
Your boat’s gunwale is designed to protect the hull from damage caused while casting off and docking. Similar to bumpers, they absorb the knocks and scrapes. The more elastic the gunwale is, the heavier the knocks it can handle. But your gunwale (or rub rail as it’s also known) is only as effective as the sealant used to bond it to your boat..
Polyurethane is the best sealant material for gunwales because it is highly elastic and also can withstand high temperatures from direct sunlight. Without an elastic sealant, your gunwale may not be able to handle the stress of constant knocking against hard surfaces. If you use screws to secure your gunwale, this sealant should also be used to fill in the drilling holes to keep out dirt and water.
There are two main polyurethane sealants on the market we recommend for gunwale.
The 3M 5200 Adhesive Sealant is extremely strong above and below the water line and most importantly stays flexible so it can handle structural movement.
Best of all you can use this sealant on many other boat fixtures including bonding your fibreglass hull and deck, portholes and deck fittings, motors on fibreglass transoms, any wood to fibreglass and under mouldings. The permanent bond on all these materials remains strong, flexible, cures within five-to-seven days and is tack-free in 24-48 hours. Applied by hand or with a pressure caulk; the product won’t sag or flow into seams.
The Sikaflex 291 Sealant is a similar product that’s both elastic and resistant to weather and ageing. Like the 3M product, it can be used above or below the waterline to bond wood, metals, metal primers, paint coatings and ceramic materials. This sealant cannot be applied to perspex or polycarbonate as these are prone to stress cracking.
Like most paint and sealant products, preparation is the key. If you don’t have a well-prepared surface, your sealant won’t be effective or will have a shortened life.
If your gunwale is made of moulded PVC or polyurethane, sand down the bonding area on it using a 60-80 sandpaper then remove the dust. Remove any grease or dirt from the hull of your boat.
Apply a thin continuous coat of primer with a clean brush or applicator. Allow to dry for a minimum of 30 minutes but not more than 24 hours.
1. Apply sealant to the bond area in beads of approximately 2mm in thickness
2. Assemble components within 20 minutes of application
3. Press gunwale into place, on to the face or over core profile
After the Gunwale is Installed
4. Use clamps or masking tape to hold the gunwale in position while the sealant sets
5. If mechanical fixings have been used, fill any holes with sealant
6. Remove clamps or tape after 24 hours
7. Full strength is attained after seven days (or 24 hours if using the 3M 5200 Fast Cure Sealant)
8. Any uncured sealant can be removed with Sika Remover or mineral spirits
For the best performing, shock-absorbing gunwales, apply a good quality sealant properly. Your boat will be able to weather years of knocks and scrapes without needing to be replaced.
If you have any questions about sealants, don’t hesitate to contact our experienced team at Boat Accessories Australia on 1300 308 161 or send us an email.