Choosing Wood for Marine Applications
Navigating the Bewildering Array of Possibilities
Strong, Stable, Superlative
Among the materials available to the modern mariner, none is more versatile than epoxy. For gluing, repairing, reinforcing, finishing – even for molding mechanical parts – epoxy provides a unique solution. It’s the duct tape of the sea, only far better. Epoxy can enhance the quality and durability of your boat in innumerable ways, can replace and outperform many other traditional materials, and it’s easier to use than you might expect. But before we get to the details of why and how to use epoxy, we need first to know what it is.
Around 1920, chemists started playing around with the polymers (plastics) from which epoxy would eventually be developed. The formulation was complete by 1946, when epoxy – originally called Araldite – was made commercially available. It has been steadily growing in popularity for over fifty years. Today, epoxy is used widely in the marine, automotive and aerospace industries: the wing skins of fighter planes are made from it, your backyard patio furniture is probably coated with it, the rocket fuel of the space shuttle is stabilized with it. Like the polyester resins used in fiberglass construction, epoxy is one of a number of complex chemicals, derived from natural gas and petroleum, that we take for granted but which provide great benefits.
Look through the tool cabinet in your garage or the lazaret where you keep all those cans of glue and patching compound and waterproof filler – you know, the place where all the junk is. You can toss out most of that gear (it’s probably dried-up anyway). Epoxy will replace all of it. But keep your supply of polyester resin. Except under exacting conditions, epoxy does not perform well when applied over polyester (and polyester never performs well over epoxy). For everything else, consider epoxy your one-stop solution.
Most ship chandlers and marine retailers sell good-quality epoxy. The West System and Industrial Formulators are two popular brands. The type of hardener makes a great deal of difference in the final quality of the product, so be sure to purchase epoxy from a reputable supplier. Avoid the hardware store epoxies with their disposable syringes; these are not nearly as effective as the quality brands.
Epoxy comes in a two-part format, resin and hardener. Try to find a brand that supplies dispensing pumps on the top of each container, one pump for the resin and one for the hardener. This makes mixing easier and more exacting. Some formulations require a special hardener – quick-setting, or with extra clarity for wood finishing – but the default hardeners work well in most applications. Store the resin and the hardener at room temperature. The shelf-life of epoxy is about two years, though sometimes a process called thermal cycling causes the liquid resin to crystallize. It doesn’t cure, it just gets cloudy and begins to solidify. There’s nothing wrong with the product; just place it in an open area, heat a kettle, and waft it with some steam. At about 50 degrees Celsius (122 degrees Fahrenheit), the epoxy will liquefy again. Make sure the container is sealed tight if you need to follow this procedure, as one of the only ways to ruin uncured epoxy is to let it come into contact with water.
Take a walk around your boat and notice all the things coming loose: stanchions, wood joints, hardware on the dinghy. Fix it all with epoxy. As an adhesive, epoxy is far superior to anything else: it’s virtually impervious to moisture and decay, is stronger than any other kind of glue, will last longer, and it doesn’t give off unpleasant, toxic fumes. But wear gloves: epoxy can cause allergic reactions on the skin.
Before applying epoxy, use a solvent to clean the surfaces being affixed. If you’re working with teak, the solvent should be acetone. Let everything dry, or nudge the work along with a hairdryer (unless you’re standing near water!), then sand the surfaces with 80 grit sandpaper. Wipe off the dust and you’re ready to go. Through a process called keying-in, epoxy migrates into the sanding scratches and other irregularities on the surface. These are what make the bond, so be sure not to skip the sanding step.
You will see on the epoxy package directions (which you inspect diligently, right?) that the resin and hardener should be combined in the correct proportions, then mixed together for about two minutes. This is a crucial step. If the constituents are not sufficiently mixed, layers within the epoxy will not cure and the adhesion will fail. This situation is one proposed cause of the crash of the space shuttle Columbia, in which foam insulation adhered with epoxy came loose and struck the heat tiles of the left wing (also affixed with epoxy).
Once mixed, the epoxy will remain workable for a long while compared to other adhesives. Depending on the hardener you use, and the ambient temperature, the liquid state may persist for as much as an hour. If it starts to thicken into a gel before you’re ready, spread it out onto some newspaper.This cools the mixture and slows the thermosetting process. Take your time. Apply the epoxy with a brush, as a thin film, then ease the parts together (with clamps, if required, but clamp lightly). Wipe away any excess with a rag and some rubbing alcohol. Other solvents work well also, but alcohol is not as toxic. Let the adhesion set for24 hours until the epoxy has cured (full curing actually takes a few days, but most of the strength is achieved after 24 hours).
If your boat has a wooden hull or a wooden deck, and if it’s more than a few years old, you’ll see some wood decay. The fibers of the wood tend to crumble and turn black on exterior surfaces, whereas internal surfaces become cracked and soft. Dry rot is not actually dry. Instead, it results from trapped moisture in areas of poor ventilation. The closed interior of a boat is a prime location for such decay.
Wherever you see wood decay on your boat, inside or out, use epoxy to repair the damage. It’s a simple process: remove all the decayed material and fill the void with epoxy. If you prefer, you can thicken the epoxy with various additives (available from the manufacturer) to ease the application and strengthen the bond. Some epoxies have a maximum application depth of about half an inch, so more than one coat may be required for large voids. If you want to match existing wood, you can use a colored additive, wood dust, or regular paint tints mixed with the epoxy. All work well, though paint tints, which are available in small tubes from paint stores, offer the most exacting solution.
If you want to reproduce the profile of a decayed gunwale edge or the corner of a cockpit door, apply masking tape to the wood to create the intended shape of the void and pour the epoxy in. Small syringes can be used for this purpose. After the epoxy cures, sand the surface to precisely match the desired contour. If this type repair is done carefully, if the color matching is accurate, a casual inspection of the wood surface will not reveal that any work was done.
Epoxy is strong enough that it can be used to affix a keel – not that you’d want to rely on it as your sole means of fastening, except on a dinghy. (The keel on my own dinghy is secured with an epoxy fillet. No fasteners means no leaks.) Hardware mounts that combine epoxy with fasteners offer tremendous durability and mechanical strength. The fasteners carry the single-point loads and distribute them across the surface area of the epoxy. Using this technique, you can dramatically increase the load-carrying capacity of your hardware components. Here’s how it’s done:
Drill large-diameter holes, as much as twice as wide as each screw, but only drill to about two thirds the required depth. Drill regular pilot holes, the screw diameters or slightly less, into the deepest third of the holes. Sand the base of the hardware and the contact surface with 50 grit sandpaper.Mix a batch of epoxy, using a filler if desired (the West System 404 filler, for example).Fill the holes with epoxy, coat the screws, and coat the hardware mounting surfaces.
Place the hardware, insert the screws, and tighten everything down. The screws will grab the bottom third of the holes and snug the hardware to the mounting surface.(For a more complete description of this procedure and suggestions for specific applications, see the Gougeon Brothers guide Wooden Boat Restoration and Repair, ISBN 1-878207-24-5. Also, some epoxy manufacturers recommend against using stainless steel fasteners in this scenario, as the epoxy deprives the steel of oxygen, which is required for corrosion resistance.)
So far, we’ve replaced all the adhesives in your tool cabinet. All the fillers and patch kits as well. How about the finishes? Previously, I have discussed epoxy as an ideal wood finish, but it bears repeating that epoxy offers the most durable wood finish achievable by any means. If you coat the cured film to protect against ultraviolet light (with polyurethane, for example), your wood surfaces will gleam for years to come. I’ve also used epoxy to recreate the missing parts of broken carvings, to reseal cracks in damaged metal, to fabricate missing mechanical parts. And, of course, it can be used in the same way as polyester resin: to wet out sheets of glass fiber in the making of hulls and panels. But fiberglass with epoxy has, at minimum, twice the strength of fiberglass with polyester (and as much as seven times the strength).
However you use epoxy – as adhesive, finish, or the ultimate patch-kit – remember to follow three basic rules: buy quality, wear gloves, mix well. The rest is just practice, and imagination.
First, the basic facts: epoxy and polyester are both polymers (heterochain polymers, to be precise).They are cousins within the same chemical family. Polyester is the resin used with fiberglass in the vast majority of boat hulls. Epoxy is gaining in popularity, especially as a composite material with wood, but its cost makes it prohibitive to some boatbuilders and buyers. Vinylester, a kind of hybrid between polyester and epoxy, is not yet widely used in boat building.
The tensile strength of Polyester is quite good: about 20 to 70 MPa (MegaPascals). But on the same scale, epoxy rates between 35 and 140. This is the main advantage of epoxy; it’s much stronger than polyester, and slightly more flexible.
Both epoxy and polyester can provoke skin allergies, but polyester poses the added danger of fumes: out-gassing from styrene, and reaction to the hardening agent, methyl ethyl ketone. I once worked in a ship chandlery where a co-worker was rendered unconscious by accidentally inhaling the fumes from a small leaking tube of MEK. Epoxy, conversely, gives off a slight, rather pleasant smell.
Cured epoxy resists almost all acids and solvents, except those with chlorine. Polyester is not typically resistant to solvents but handles acids fairly well (though not sulphuric acid).
Epoxy costs more than polyester; way more. This is its chief disadvantage. Good quality epoxy can be three times as expensive as polyester. For the mariner, the negotiation between cost and quality is an old one. Usually, the boat wins.