The Dacron/Xynole/Epoxy Matrix
By Ross Miller - West Mystic, Connecticut - USA

Light bulb moments are few and far between for me, but I experienced one a few years back when it occurred to me that Xynole polyester needn’t be limited to sheathing plywood boats, that it could easily be used as a second layer over a heat-shrink Dacron skin kayak. Maybe it was more of a Homer Simpson moment, a bout with the obvious, but however one might regard its conception, the Dacron/Xynole/Epoxy Matrix©, as I have pompously named the method, has turned out to be a good thing.

Skin-on-frame kayaks have come a long way since the days of seal hides, but the search for a tough, durable and manageable covering continues. Canvas was the material of choice when non-arctic people first realized what a triumph of ingenuity kayaks were and began to build them for themselves in locally appropriate ways. Since then, materials have ranged widely. On the frivolous-to-dubious end of the spectrum one will occasionally find varnished paper or a blue tarp. On the other end are the strongest fabrics, such as ballistic nylon, which provide skins that can be dragged over mussel beds or lava with no ill effects, or so it is claimed. And then there is aircraft heat-shrink Dacron. I make no secret of my fondness for heat-shrink Dacron, since it is light, strong, and easier to manage than any other covering I have tried.

Nevertheless, many people seem wary of heat-shrink Dacron, fearing that a fabric so light will be reduced to tattered shreds if one merely looks at it the wrong way. This is just not true - Dacron is very strong for its weight - but reinforcing Dacron with a layer of Xynole and epoxy is an attractive and viable way of making a very strong covering using materials that are easy to work with. Xynole is an open-weave polyester laminating fabric that is available only (as far as I know) from Defender Industries. Its greatest advantages are its strength and its ability to drape gracefully over difficult curves. An explanation of the method follows.

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First the frame is covered with the heat-shrink Dacron.

First the frame is covered with the heat-shrink Dacron. In a wind-free environment, drape the Dacron over the overturned hull, leaving six or more inches of excess at each end. Align the warp of the fabric parallel to the keel. Pin the fabric to the keel at the outermost ends where the keel meets the stem and stern pieces. Slit the fabric down the faces of the stem and stern pieces, starting a little fore and aft of the pins. Begin gluing the fabric to the sheer stringers amidships, first one side and then the other, alternating toward the ends to keep it evenly spread across the hull. (The glue is a contact cement that can be obtained, along with the Dacron, from Aircraft Spruce, a supply house for aircraft builders.) Press the fabric into the glue along the stringers so that it adheres well. You do not need to pull the fabric too tightly (the heat will do that) but it should be glued as evenly as possible over the frame.

When it’s glued along the sheers to the ends, spread glue along the forward face of the stem piece. Pull one side of the slit cloth across the stem and press it well onto the stem. Allow it to dry, trim its edge to about 1/2 inch from the stem, then repeat for the other fold. Do the same at the stern.

Now go all the way around the sheer and ends and make sure that the fabric is glued securely. If there are any gaps, brush some more glue into them and press firmly. When all is dry and secure, you are now ready to shrink the hull fabric. I have found it best not to trim the excess along the sheers until after shrinking, because the skirt gives you something to hold on to should any spots come unglued during the shrinking process and need regluing.

The shrinking is done with a common household iron. Personal experience leads me to advise against a heat gun because the distribution of heat is too inconsistent, and also because a moment’s inattention can lead to burning a hole right through the fabric. It’s a clean job, so if the builder needs to borrow the iron from a domestic partner, it can be done openly and with a clear conscience.

Set the iron at 250F (medium or slightly less; irons vary) and begin the shrinking. Start at the center of the boat’s bottom and work outward in circles toward the end, doing the sides, too, as you progress. The Dacron will shrink gradually, and you should pace the process so that you cover the entire surface of the hull with the iron more than once before it’s taut. And don’t overdo it, either, because the layer of Xynole will add plenty of stiffness. You don’t want any bagginess anywhere (make sure there is concavity at forefoot and sternfoot) but don’t shrink it so hard that the stringers are bent inward. It can be done, and you don’t want to. It scrunches the lines of the boat.

When the shrinking is complete and you’ve gone over every inch of the glued surfaces and made sure there are no gaps, it is safe to trim the skirt. Use a good sharp blade. My tool of choice is the old Case pen knife, freshly honed, that was my father’s. Once you’ve removed her skirt, she’s ready to be turned upright and admired.

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Once you’ve removed her skirt, she’s ready to be turned upright and admired.

Covering the deck is much the same. Align, leave some extra at the ends, and pin at the ends, overlapping the hull fabric. Glue along the sheer as before, then at the ends tuck the sides under the forward flap, overlapping the hull fabric by up to an inch, stuff full of glue, and pin until dry.

The cockpit opening is cut by slicing the fabric, first fore-and-aft then athwartships, keeping the cuts several inches away from the carlin to begin with. Make four more slices between the first ones, and probably four more, too. This will vary depending on the shape and size of the cockpit. Start clamping the Dacron to the carlin with spring clamps. Experimentation will show how much the slits have to be enlarged to allow the cloth to conform to the carlin. The farther the slit ends are from the top of the carlin, the better. When the opening in the fabric is sized to the carlin satisfactorily, glue it, starting with the ends.

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When the opening in the fabric is sized to the carlin satisfactorily, glue it, starting with the ends.

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Once again, check all around to make sure the gluing is uniform, then shrink the Dacron as was done with the hull, and finally trim the skirt. Special care must be taken when trimming the excess from the deck fabric to make sure your blade does not slice through the hull fabric beneath. That would ruin your day, if not more.

The Dacron part of the process is finished. You might be tempted at this point by the airy lightness of the Dacron-only kayak to forgo the additional strength of the Xynole layer, and that would be a valid choice, but this article does not have that option because it is titled “The Dacron/Xynole/Epoxy Matrix” and so must go on to describe the application of this fortifying (and beautifying) second skin. Anyone who has sheathed a plywood or strip-built boat, or has seen the process described, is already familiar with the underlying technique, although there are some variations unique to its use on skin boats.

Begin as before with the boat upside down. Make sure whatever it is resting on is well cushioned, because the unfinished Dacron of the deck is prone to tearing at this point. Even the tiniest snag will disrupt the weave of raw Dacron when (not if) the boat is jostled.

The first step is to wet out the hull Dacron with unthickened epoxy. A 3” foam roller is the best size, and a clean plastic microwave dinner tray holds a usable amount of epoxy. Apply the epoxy evenly to the fabric. The aim is to saturate the fabric but no more, so don’t overload the roller with epoxy or overlap areas that are already wet. You don’t want it pooling on the inside of the skin or dripping onto the woodwork. Slow-cure epoxy is recommended because it gives you the most time to work and also a better chance of achieving the best bond.

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Slow-cure epoxy is recommended because it gives you the most time to work and also a better chance of achieving the best bond.

It is ideal to apply the Xynole when the initial epoxy is at the green-cure stage, i.e. not yet hard but no longer sticky. It can be applied later if this is not possible, as long as blush has been removed and the surface is clean, but the advantage to a green-cure wet out is maximum adhesion. Trying to apply the Xynole while the epoxy is still sticky is dicey and not recommended. Align the weave of the Xynole and pin at the ends of the keel, pretty much as was done with the Dacron. Don’t slit the ends yet, as it will be more apparent later how they will lie.

The Xynole is wet out with unthickened epoxy. Pour some onto the center of the bottom and spread it outward with an autobody squeegee. Apply judiciously, just enough to saturate the fabric and make it adhere to the Dacron; filling the weave will come later. Spread the epoxy evenly, making sure that there are neither dry spots where the fabric layers are not joined nor pools of excess epoxy.

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As you begin to approach the ends (not at the last moment) take the time to plan how to slit and overlap the ends.

As you begin to approach the ends (not at the last moment) take the time to plan how to slit and overlap the ends. It will differ from boat to boat depending on the shapes of the ends, but the principle is similar to the Dacron. Xynole is thicker than Dacron, so small bumps will be inevitable where the slits end and the overlaps begin, but they will be faired later, so just try to keep them pinned down as best you can while the epoxy cures. Check all around (especially the sheers) to make sure adhesion is complete. Any area where the Xynole is not joined to the Dacron will cure into an unsightly hump, so be thorough and keep checking as the cure progresses (especially the sheers) because sometimes it will pull away when your back is turned.

It is best to trim the skirt just as the epoxy reaches the green stage, stiff enough not to pull away or stick to the blade, but soft enough to cut through easily. Cut a smooth line right along the hull-deck joint, once again taking care not to cut through any other fabric. Leave enough at the ends to complete the curve of the sheer.

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Now is the time to for the final decision as to whether the deck will be sheathed in Xynole, too.

Now is the time to for the final decision as to whether the deck will be sheathed in Xynole, too. It will be lighter without, or stronger with, the additional laminate. If you’ve elected to continue, it’s best to do so while the epoxy is still green.

When it’s no longer sticky, set the boat upright. There will probably be gaps between the Xynole and the Dacron at the tops of the stem and stern, and these gaps should be filled with a syrupy mix of Cabosil-thickened epoxy to form a solid impact-resistant mass. Any other gaps that escaped previous inspection should also be filled before the deck fabric is applied. Make sure the Xynole is trimmed smoothly along the sheer and at the ends.

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Gaps between the Xynole and the Dacron at the tops of the stem and stern should be filled with thickened epoxy to form a solid impact-resistant mass.

Drape and align the Xynole over the deck and mix more unthickened epoxy. Spread as before. Another fluorescent moment was when I thought of applying masking tape 3/4” below the sheer line to assure a smooth border between the hull and deck layers, but I haven’t had much luck getting the tape to stick, so some care is required when gluing the overlap to keep it even. There is, of course, no reason to wet out the cloth over the cockpit opening. Right up to the edge and maybe a little past is best. When the deck fabric is at the green cure stage, trim the excess and cut out the cockpit opening.

Now it’s time to fill the weave with thickened epoxy. Use epoxy thickened with Cabosil to the consistency of mayonnaise. Cabosil is used in this first layer because it is hard to sand, and thus protects the Xynole, which becomes annoyingly fuzzy if exposed by sanding during finishing. Subsequent filling layers (until the surface is smooth) are done with microballoon-thickened googe, because it does sand easily.

Spread the Cabosil mayo with an autobody squeegee, thinly but thoroughly. You want to cover every square millimeter of the boat (first hull, then deck, as before) lest the dreaded fuzz ruin an otherwise perfect finish. Spread it as smoothly as possible, eliminating as much as possible the little ridges that trail from the edges of the squeegee. You can fair (fill) in the small step between the deck and hull fabric, but you don’t need to make it disappear this time around (if ever) since the rubrails will cover the joint. Work just enough along the edge of the deck fabric to bridge the gap and make both layers one.

When the Cabosil layer has semi-cured, go over the entire surface with your hand to find any ridges or other irregularities. Scrape them smooth with a cabinet scraper, or with a chisel (held upright with the fingers like a pencil) or with a penknife held sideways. Be careful.

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Then mix up some microballoon mayo. It is with this variety of googe that the weave of the Xynole will be filled to smoothness.

Then mix up some microballoon mayo. It is with this variety of googe that the weave of the Xynole will be filled to smoothness. Do the entire hull as before, applying as thin a layer as possible. It will take two layers to fully cover the weave. Don’t try to do it in one, because it’s hard to sand excessive accumulations from the flexible fabric. As with painting, thin coats are better. Build up a bead of epoxy along the edge of the chine to make a protective corner. It will be sanded smooth and round.

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Build up a bead of epoxy along the edge of the chine to make a protective corner.

The stem and stern are faired with microballoon epoxy, too. File down the humps where the fabric crossovers pucker a bit, but not so much that the fabric is compromised. This time use a fairly thick mix, thick enough not to sag. Build the epoxy out somewhat from the ends, then fair it back to the proper profile when it is hard.

The stem and stern are faired with microballoon epoxy, too.

When the microballoon layer has cured completely, sand it smooth with 100 and then 150 grit sandpaper. This is a hand-sanding job; over-sanding might raise the fuzz. Caress the surface with your bare hand to find irregularities the eye can’t see. A good job of sanding will provide a smooth substrate for a flawless paint job.

And there you have it: the essence of my skinnovation, the Dacron/Xynole/Epoxy Matrix©, in all its revealed glory. People have asked if I have patented this process. I have not. Even if this particular type of lamination could be patented, it is not my aim to prevent people from using it. Rather, I would like to see the method proliferate, preferably on boats of my own design, though it’s also a good way to bring an old frame back into use. I’d also be interested to see one of Platt Monfort’s Geodesic© sailing dinghies covered in this manner. Please go ahead and try it. Just don’t forget who told you.

Other articles about skin-on-frame kayaks:

SAILS

EPOXY

GEAR