Sealing deck fittings
Rupert Holmes explains the common causes of leaky deck fittings and shares his tips to achieve a perfect, long-lasting watertight seal
How to ensure your deck fittings don’t leak
An effective seal between the deck and fittings is essential to prevent water finding its way either directly into a boat’s accommodation spaces, or even worse, into the foam or balsa core of the deck moulding.
Almost every fibreglass boat – even many of the very earliest – has a foam or balsa cored deck, which will degrade or delaminate if wet, significantly weakening the structure of the vessel.
Yet all too often the sealing of deck fittings and other items such as hatches are seen to need no more than a case of a quick smear of silicone from a mastic gun.
That may work for bathroom fittings that are never flexed under load, but on a boat it’s an approach that can store up significant problems for later.
I suspect many owners don’t fully appreciate the difficult job sealants have to do. One problem is that degradation caused by ultra-violet light will break up the edges of the sealant. A more fundamental problem, however, is that the sealant may not adhere well to either the deck or the fitting. Once the adhesion is lost, then the water is free to find its way around the sealant, and can even be encouraged to do so by capillary action.
An often bewildering array of products can also make it difficult to confidently choose which one to use for different purposes. This is probably not helped by manufacturers who tend to give products apparently obscure code names.
A further concern for many owners is removing a fitting at a later date. This worry often leads to many using a silicone sealant. However, as already said, these rarely fare well in the long term in the marine environment.
The biggest drawback is that, once cured, they have poor adhesion. This means that once a boat starts in flex in a heavy sea, or under its rigging loads, the bond is easily broken, allowing water to seep between the boat and the silicone.
Many boats, especially older examples, unfortunately have deck fittings, windows, hatches and other items sealed with inappropriate products. Having bought vessels like this in the past I’ve spent an age unbolting and refitting deck gear, stanchions and even toerails, before fixing
the damage done by water ingress. The latter of course, is complicated by the fact that the location where water gets in is rarely where you find it – it can travel a long way behind headlinings and the like.
With proper planning few deck fittings will ever need to be removed, so why not use a decent polyurethane (PU) marine grade adhesive sealant? This is the same class of sealant that’s typically used to bond the windscreen of a car to its bodywork. It’s a single component material that’s strong, easy and quick to apply, durable and resistant to UV, diesel spills, cleaning products and so on.
Even better, it retains elasticity once cured, which helps absorb the movement between the boat and fitting that would cause a solid sealant to crack and fail.
Sikaflex 291i, or 3M 5200 are most commonly found in my toolkit. These are general purpose products that are suitable for almost any sealing and light bonding task on wood, metal, plastic or ceramic items on board. They can be used for all deck fittings, plus underwater items from skin fittings to the keel.
If you know a deck fitting will need to be removed at a later date there are a number of options. If they are on a reasonably flat area of deck, many fittings can be cut off, cheese wire-style, after removing the bolts – this is by far my preferred option. Otherwise there are lower-strength polyurethane products formulated for easier removal of fittings.
Alternatively another class of sealant – polysulphides – have good adhesion like PU products but don’t cure, which allows for easy removal. Either polyurethane or polysulphide sealants will generally produce a much longer lasting seal than any silicone product.
An further option is butyl tape, which is used extensively in the domestic glazing industry. This should be compressed to around half its initial thickness. It can be used for windows and hatches, as well as other types of deck fittings and is a lot less messy to work with. I’ve used it to good effect with windows and would do so again, but I still prefer to stick with polyurethane for other deck fittings.
Surface preparation
It comes as a surprise to many that a product as sticky as a PU adhesive sealant – that will all too easily attach itself to anything and everything that gets in the way, including clothing, skin and ropes – needs careful surface preparation.
This perhaps explains why preparation is a vital part of the process that is all too often skimped – even though doing so will
inevitably lead to an unnecessarily early failure of the seal.
“Surface preparation is absolutely essential – it’s impossible to emphasise this enough,” says Gareth Ross, who’s responsible for Sika’s marine products.
“Without the correct surface preparation the product is essentially useless.”
The need for thorough preparation is particularly true where a silicone sealant has been used in the past – if there is any residue left a new sealant of any type will not adhere well.
Unfortunately, removing all traces of old silicone can be a big challenge. A chisel or knife will help to clean a lot away, but a proprietary silicone remover is often a good idea to eliminate all residual traces.
It’s important to do this before sanding to key surfaces – otherwise you just spread the contamination around.
The primary aim of surface preparation is to remove all contaminants, whether they are oil, saltwater, dirt, dust, greasy fingerprints or guano.
Alcohol is an effective cleaner in this context and will leave a good surface. Sika’s Aktivator 205 goes one stage further – once the alcohol evaporates it leaves an adhesion promoter that improves adhesion by around 20%.
If bonding without mechanical fixings, Sika Multiprimer Marine is a clear primer which dries in 30 minutes, leaving an optimal surface on all materials other than glass and one-pot painted surfaces.
The next step in making a neat job is a dry fit so you can accurately mask around the edge of the fitting and the matching part of the deck. This allows you to subsequently cut away any excess with a knife when it has mostly cured.
Application
Don’t worry about putting too much sealant on – many of the leaky fittings I’ve encountered clearly never had a full seal around the bolt holes when they were first installed. If the sealant doesn’t squeeze out all round the fitting you have no way of being certain that it is properly sealed.
Use a line of sealant that encircles all the bolt holes, applying enough so that it will be squeezed out along the full length of the join, but not so much that there’s excessive overspill.
Try to keep a steady hand, with a consistent pressure on the trigger of the mastic gun and the nozzle moving at a steady pace.
A common cause of problems in this respect is using sealant that is beyond its use-by date – this is harder to expel and may therefore require considerable pressure. A quality gun, maybe even an electric model rather than a £3 DIY store
version, also makes it easier to apply an even bead of sealant.
It’s important to recognise that some fittings have specific channels for sealant, while others have spaces that must be kept free of sealant. Winches are perhaps the best example of the latter – if the (usually three) drainage channels at the bottom are blocked the winch will tend to fill with water over time, with unfortunate consequences for the internal workings.
A companionway hatch garage is another prime example of an item with drainage channels that should not be blocked with sealant.
Remember that a 2-3mm thickness of sealant is optimal. In some cases the bottom of deck fittings are sculpted to allow for this, but it’s more likely you will need to guard against overtightening bolts. It may be worth using spacers to ensure you don’t squeeze too much sealant out of the joint when screws or bolts are tightened.
While PU adhesives don’t contain any particularly hazardous chemicals, in common with other products excessive contact with the skin is not a good idea. This makes a ready supply of disposable gloves a sensible precaution.
Bonding
The marine industry was an early adopter in adhesive technology to avoid, where possible, the need to use mechanical fasteners such as screws, bolts and rivets.
The advantage of this is reduced assembly times, which make boats more affordable for their first owner, plus an absence of unnecessary holes that can create water ingress problems for owners who come along later.
A triangular bead – achieved by cutting a V in the nozzle and holding it at 90° to the surface – improves surface contact when bonding items together. When you press the two objects together the top of the bead collapses in to give a better distribution of sealant, whereas a rounded bead will tend to leave areas at the edge of the bead with no contact with the substrate.