Building A Pack Canoe - Part 5: Applying The Fiberglass And Epoxy Resin

OK... I messed up by not taking any photos of the outer fairing process. Fairing is the procedure for making the hull of the boat smooth and free from bumps and hollows. No matter how carefully the strips are glued together, there is a little ridge at each strip juncture due to the fact that the strips are flat and they are going around a curved shape. The result is that the surface of the canoe is composed of dozens of facets that all need to be very carefully blended together to get a smooth skin.

We used many tools to perform this operation. First, we used a fine rasp to break off the excess glue that oozed out of each joint. Then we used a small block plane to cut the apex of each ridge and finally we used a combination of cabinet scrapers, power sanders and hand sanding to get the hull surface perfectly smooth and fair. This is just about the most exacting part of strip building, and with a strip thickness of only 1/8" in places, it was imperative that we take our time to avoid sanding right through the hull in spots. This is also the reason why we took so much time getting the strips as level with each other as possible when we were applying and gluing them.

Since the boat is primarily convex on the outside, bumps are what are sought out and hollows are left untouched until the last sanding step. We spent as much time inspecting the surface for bumps as we did scraping and sanding. The job was done when we could see no irregularities anywhere no matter where we placed the work light. At this point we used a paste wood filler to fill all of the holes and gaps, let it dry and then sanded the hull smooth. We then used a shop vacuum with a brush attachment to clean the surface of dust and we were ready for applying the fiberglass.

We cut off 12' of fiberglass cloth (3.2 oz./yard) which left 6" at each end. The cloth was draped carefully over the inverted hull and smoothed out, leaving the excess hanging below the hull.











We used a 4" bristle brush to push the wrinkles out of the cloth. As we brushed, not only did the cloth lay on the boat wrinkle free, it actually clung tightly to the hull surface. In this photo, you can clearly see the wood through the fiberglass cloth.

At this time, I turned on two electric heaters to warm up the room to about 80ºF and let the boat sit in the warm room for several hours to heat up. The idea was to keep any air that got trapped in the wood or under the cloth from expanding and creating bubbles. After we wet out the boat, we turned the heaters off to allow any trapped air to contract, thereby preventing any bubbles from forming. This also allowed the exposed wood to soak up
more resin as the hull cooled overnight.

This is how tightly we got the cloth to lay against the hull by brushing it with a dry brush. This would be critical to the ease we later enjoyed when we wet the cloth out with epoxy resin.

We used System Three Clear Coat epoxy resin with the slow hardener, which gave us plenty of time to work the resin before it began to harden. Clear Coat was recommended by many experienced builders as a wet out coat. It is a high quality, low viscosity laminating resin that readily soaks through the cloth and into the wood below. Since the fiberglass is composed of real glass fibers, the cloth virtually disappears once the resin is poured on.

Here is a shot of Lady BK applying the resin onto the hull. We began by thoroughly mixing up a small batch (6 oz. resin to 3 oz. hardener) and pouring it all at once onto the surface at the center, then spreading it downward and out toward the ends with a 4" chip brush. The less resin you use, the closer the cloth will adhere to the hull, making it both stronger and lighter. However, since the resin will soak readily into the wood - particularly porous woods like basswood - care must be taken to make sure that there are no chalky looking areas that indicate the the cloth is starved of resin.

We kept applying more batches of resin and spreading it out until the entire surface looked uniformly shiny. Then, using specialized soft rubber squeegees, the excess resin was scraped from the surface until the entire boat had lost its shiny look and had a dull sheen to it. Shiny areas mean there is excess resin which must be removed. Excess resin means the cloth is partially floating within the resin and not stuck firmly to the hull. Care must be taken to get every bit of resin removed from the surface before it hardens, but you also have to continue to watch for chalky looking areas that indicate that there is too little resin for good adhesion.

The resin/hardener combo we used hardens in 9 hours, but we gave it 24 hours to get hard enough to sand. We sanded the hull enough to knock off the gloss and then applied a second coat, this time with the regular (more viscous) System Three resin. A third coat was applied in the same manner, but more heavily that the second coat. This coat - known as a "hot" coat - was allowed to flow in a continuous sheet and onto newspaper that had been placed beneath on the floor.






After another 24 hours we began to level the epoxy coat with a sharp cabinet scraper. I can think of no more effective way to remove partially cured epoxy than with a cabinet scraper. It is simply a square piece of spring steel (an old hand saw blade is perfect) that is honed to a 90º edge and then a small hook is burnished on the edge with a smooth screwdriver shaft. When bent between the hands and held at the correct angle, it removes wood or resin just like a plane does - in thin shavings. The beauty of it is that it can be used when the resin is too soft to sand. Partially cured resin gums up the sandpaper quickly. For the sanding that was done in between resin coats, I used two entire sheets just roughing it up for the next coat. On a safety note, partially cured epoxy releases fumes into the air when heated through sanding, so you need to wait until it is fully cured before you power sand it.

This is a close up of a hand scraped area. Note the ridges left in the surface. It is almost impossible to get epoxy to flow out without irregularities in the surface since it is so thick. All I was trying to do at this point was to remove as much of the high spots as possible while the resin was still somewhat workable. There would be more epoxy and glass work on the outside later, so for now I just wanted to get it relatively smooth and then finish up the final sanding after a full cure had been achieved a few weeks later.





Here I am using a RO sander (slow speed) to do a quick blending of the surface left rough by the scraper. I should have been wearing a respirator, but as you know by now, I can sometimes be a bit lax when it comes to safety procedures. Even though the System Three products I was using are considered very low in toxicity, I strongly recommend a chemical respirator if you are sanding uncured epoxy.







Finally, the hull is off the form. I cut off the excess resin-soaked cloth with a 2" slick. It peeled off like butter under the mass of the heavy tool. The individual section molds were then detached from the strong back and knocked toward the center of the boat to free them from the hull. I decided to see how the weight was doing, so I improvised this setup to check it out. Subtracting the weight of the support stick and string, the hull weighed 11 pounds, 11 ounces.

Earlier calculations had led me to believe the hull might weigh as little as 10-10 1/2 pounds at this point. I had mixed up 36 oz. of resin in total and squeegeed off a weighed 7 oz. With a surface area of about 29 square feet, there should have been between 9.5 and 12 pounds of wood in the stripped hull (depending on the actual density of the wood) minus what was faired off (maybe 5-8%, or about 8-12 oz.). 29 sq.ft of 3.2 oz. cloth should have weighed about 10 oz. A 50/50 ratio of cloth to resin by weight is considered good laminating technique, so there should have only been about 10 oz. of resin in the glass at this point plus whatever soaked into the wood. I ended up using an extra 19 oz. due to the absorption of the resin by the very porous basswood. This was unacceptable when trying to build an ultralight boat, but I had to live with the fact that the boat would weigh a full pound more than I had hoped... and I still had to glass the inside. Future boats will be made from much lighter and less absorbent northern white cedar.