Short answer
A leaking fitting should be removed and rebedded. Running a new bead of sealant around the edge rarely lasts. Unscrew the fitting, remove all the old sealant, clean and dry. Apply new sealant under the fitting and tighten in two stages. Also check that water hasn't reached the deck core.
Find the leak
- Water runs along the underside of the deck. The drip often appears some distance from the leak.
- Look for sealant that has come loose, gelcoat cracks around the fitting and rust or brown streaks under rails and stanchions.
- Hose one fitting at a time while someone watches from inside the boat.
- Tap around the fitting with a screwdriver handle. A dull sound can mean the core is wet.
Which sealant?
| Type | Suitable for | Keep in mind |
|---|---|---|
| Polyurethane, e.g. Sikaflex-291i | Fittings meant to stay put for a long time, joints that move | Bonds hard. Difficult to remove next time. |
| Butyl tape or butyl sealant | Fittings you want to be able to remove again, e.g. stanchion bases | Does not cure. Tighten several times until it stops squeezing out. |
| Silicone | Nothing on deck | Bonds poorly and leaves a film that stops new sealant from adhering. |
Sikaflex-291i is a one-component polyurethane that cures with moisture in the air. It can be used between 10 and 40 °C, ideally between 15 and 25 °C. It skins over after about 60 minutes at 23 °C, so you must be finished with the fitting and the bead within that time. For acrylic there is a separate product, see replacing a boat window.
What you need
| You need | For |
|---|---|
| Screwdriver and spanners, ideally a helper inside the boat | Undoing the nuts below deck |
| Plastic scraper, knife and thin wire | Freeing the fitting and old sealant |
| Sika Remover-208 or another solvent | Removing uncured sealant from tools |
| Masking tape | A clean edge around the fitting |
| Countersink or drill | Chamfering the hole in the gelcoat |
| Sealant and caulking gun | Sealing |
| Epoxy and thin syringe | Filling holes in cored decks |
| Nitrile gloves and safety glasses | Protection from sealant and solvent |
Step by step
- Remove the fitting. Unscrew the nuts below deck. Cut around the base with a thin knife or wire. Do not pry with a screwdriver against the gelcoat.
- Remove old sealant. Scrape everything off the fitting and the deck. Uncured Sikaflex can be removed with solvent, but cured sealant can only be removed mechanically.
- Check the core. If the deck is cored with balsa or foam, poke a bent nail into the hole. If wet or dark material comes out, the core is damaged. See below.
- Chamfer the hole. Make a small chamfer in the gelcoat around each screw hole. This forms a ring of sealant that seals around the screw, and the gelcoat will not crack when you tighten.
- Clean. Surfaces must be clean, dry and free of grease, oil and dust. Sika refers to its pre-treatment chart for the cleaning and primer different materials need.
- Apply sealant. Apply sealant to the underside of the fitting and around the screws. Put the fitting in place.
- Tighten in two stages. Tighten until the sealant starts to squeeze out. Let it cure, then tighten the nuts below deck a little more. This leaves a sealing gasket. Hold the screw head from above so you do not twist the joint apart.
- Finish the joint. Remove excess and pull off the tape before the sealant skins over.
If the core is wet
A wet core makes the deck soft and can be damaged by freezing over winter. Let the hole dry. Then drill the hole larger, dig out the core around the hole, fill with epoxy and drill a new hole in the cured epoxy. That way water cannot reach the core again. If a larger area is soft or gives when you walk on it, have a boatyard look at it. More on this in soft deck and wet core.
Common mistakes
- Silicone on deck. New sealant won't bond where silicone has been.
- Sealing around the edge without removing the fitting. Water finds its way underneath.
- Tightening everything at once. The sealant gets squeezed out and the seal becomes thin.
- Working in the cold. Curing is slower when it's cold, because the air is drier.
- Forgetting the core. Water spreads through the deck far from the hole.
DIY or boatyard
Most people can handle stanchion bases, cleats, through-hulls and small hatches themselves. A soft deck, cracks over large areas or leaks around the mast and chainplates are jobs for the boatyard. Request a quote via Marinpedia or search under Marinas & boatyards.
Frequently asked questions
Can I just run a new bead of sealant around the fitting?
It may hold for a short time. Water usually gets in under the fitting and around the screws, so you need to remove it and reseal.
How long should the sealant cure before I retighten?
Wait until the sealant has skinned over and started to cure. For Sikaflex-291i, skinning takes about 60 minutes at 23 °C, longer when it is cold.
Can you seal in cold weather?
Sikaflex-291i should be used between 10 and 40 °C. Below 10 °C you'll have to wait or heat the space.
Can I paint over the sealant?
Sikaflex-291i can be painted once it has skinned over, but the paint is less elastic and may crack at the joint.
Sources
- Sikaflex-291i, produktdatablad, Sika industry.sika.com
- Pre-treatment chart for sealing and bonding in marine applications, Sika industry.sika.com