The Day the Carbon Kevlar, Fibreglass Cloth and Epoxy Came Together

One would expect that when you build a wooden strip-planked kayak, it is made of wood.  Well, yes—that is only partially correct.  The ¼-inch thin strips of wood form the core of the kayak.  They are like the eggshell of a chicken egg.  Unfortunately, in its natural state, the type of wood we use in building these kayaks is soft and quite permeable.  They will float in their natural state, but you have to coat them with a material that keeps the water out—not only of the kayak itself but the wood it is made from.

The early Inuit kayaks were made from whale bones and sealskin, which is naturally waterproof.  As the whales and seals left, kayaks turned to wood.  Initially they were covered in tar and pitch, or even special oils.  Then along came petrochemicals, and we get polyester and vinylester resins, and now the more sophisticated epoxy resins.  Epoxy resin is a chemical process rather than an evaporative process, so mix it correctly and it will harden.  Heat only makes it more viscous.

Epoxy station

Now, having said that, resins have very little surface strength and bend easily.  To address this in constructing a kayak, one adds a closely woven cloth (sometimes chopped mat) to the surface of the wood.  This is then saturated with the epoxy resin and you get a very strong structure.  Add a more sophisticated material like carbon fibre cloth and it gets better—but wait, there's more.  Add kevlar woven into the carbon fabric and one, after adding resin, gets a very strong structure.  You can jump on it and the ¼-inch wooden strips will not bend or break, so if you jump into the cockpit it will only flex.

Carbon fibre Kevlar cloth

Carbon fibre kevlar cloth is almost $100 a square metre, so for my kayak I have to use it very sparingly—in fact, only in the cockpit area.  The professionals use vacuum bags to draw the resin onto and through the cloth and force the cloth directly onto the wood substrate.  Well, again those bags are expensive and the vacuum pump well outside my budget.  The result is applying the resin by brush and squeegee.  It is a delicate process to get just enough resin to wet out the cloth and also to ensure no air bubbles are formed between the cloth and wood.

All of this is great in theory, and one can practise to perfect your technique, but the major variable in the whole exercise is the ambient temperature, which has to be above 11 degrees Celsius.  Here in Canberra, ACT, Australia, we have—unlike those in Europe—been experiencing the longest cold stretch of night and day temperatures since reliable records have been kept.  So each day I check the weather report and the temperature increase during the day to schedule the two epoxy days.

Weather forecast

The day arrived, but the weather forecast started badly: it is only -6.3°C but a high of 14°C.  OK, that looked positive, so let's get started.  I covered the side of the carport with a large blue sheet of plastic tarpaulin to try to keep the heat in and the wind out.  Then set the diesel space heater to high as I watched the temperature in the work area rise above 11°C and on towards 20°C.  The epoxy resin and hardener were placed over a large saucepan of slow boiling water to keep them warm.  The cloth and kevlar had been sitting in the interior of the hull and deck all night, so there were no wrinkles.

Go—bring the wood, fibreglass cloth, carbon fibre kevlar and resin together in the right proportions.  One has to work methodically and exactly, ensuring the mixture is always perfect.  Never reuse a mixing cup, and try to disperse the resin before the chemical reaction in the cup starts to heat up.  A fine balancing act.

Epoxy resin on the cloth

Both the hull and deck went according to plan, and I was able to complete the hull on day one and deck on day two before the temperature at five in the afternoon took a nosedive below zero.  Both sets got tacky within two hours, so by lunchtime I was able to apply the first fill coat and by 5 PM the second fill coat.  I trimmed the excess cloth soon after dinner, but the carbon fibre kevlar would not cut with either scissors or box cutter.  The fill layers are smooth but not too thick.  By morning most of the resin was hard, and by the expected 15 hours was quite hard.

Cloth completely wetted out

That was a big job completed.  The major lesson learnt is don't start a fibreglass project in the middle of the Canberra winter unless you have a good heated room—why not my dining room?

The cockpit coaming and internals is the next big stage of the kayak project.

Sanding the Interior of the Kayak

"Why sand the interior of the kayak? No one can see it," a friend asked.  I will sand it perfectly smooth because I will always see it.  Once seen, never forgotten.  And so started the long, arduous and very dusty task of cleaning the hot glue spots and excess wood glue from the interior of the kayak and sanding it smooth, ready for the fibreglass cloth and carbon kevlar to be added to the interior.

Hot glue spots on interior

I had finished epoxying the outside of both the hull and the deck, and that had turned out well, so I made some forms for the two parts to sit in so that I could access the interior.  They sat upturned next to each other.

Hot glue spots cannot be sanded because they melt and gum up your sandpaper, so they have to be scraped off the wood where they have adhered to the interior.  I have a number of wood scrapers that I had previously used.  There is a secret method of sharpening a scraper blade.  It has to be sharp—not like a block plane—because you use the scraper not to cut shavings off a piece of wood but to scrape the shavings off.  Consequently, one sharpens the scraper blade and then runs a steel over the sharp edge and turns the edge into a hook or burr, which forms the scraping blade.  This will ensure the scraper produces fine shavings that are of uniform thickness.  A true art learnt over many years.

Scrapers ready

One starts with the obvious spots with some easy results, but then as you get closer and closer to the final output, you see how the wood glue and hot glue has settled into the cracks.  Now, strip planking a kayak seems like an easy task: lay the strips on the forms and make sure they are glued to their neighbour.  But that is not so simple because the forms are round from the part line to the keelson, and what's more, from bow outwards to the widest part and then it narrows to the stern.

I had addressed this challenge on the outside of the hull when I came to sand that surface prior to applying the fibreglass cloth.  The difference on the interior is that the strip joins may look great on the important outside, but they may not exactly match on the inside.  Furthermore, one is dealing with a concave surface into which a flat scraper or sandpaper block will not fit.  The solution is to use one of the four scrapers I had sharpened.  The large flat scraper was good for the initial glue removal and then to scrape the raised strips down to the same thickness as their neighbour.  The smaller scraper was great on the curved sides, and what's more, with their handle I was able to get a good clean shaving from the Myrtle wood.

Using the small scraper

Next comes the dirty part: sanding the surface smooth such that the fibreglass or carbon kevlar cloth will lie flat on the surface to be wet out by the epoxy.  I put on a beanie, full breathing mask and safety glasses, but still the wood dust seemed to get everywhere.  The only solution was short bursts and then retire to wash my face and head to get the dust out.

Sanding the interior

I could only continue these sessions in short bursts, so the whole operation of sanding the interior took about seven days for both deck and hull.  Once finished, I mixed up some epoxy and wood dust to make a filler and applied that along some of the open joins—again to be sanded smooth.

Filling some gaps

Now the weather again came into play, and I had to wait for some winter days that were not foggy and had a number of hours during the day that rose above 12 degrees Celsius.

Both units ready for cloth and epoxy

The fibreglass cloth and expensive carbon kevlar were waiting to be placed in the interior and wet out to add strength to the kayak and keep the wood from getting wet.