A strip canoe or kayak has a very filigree hull made of thin wooden strips, which, statically speaking, would not yet be suitable for paddling. In this construction method, the wooden hull has only a form-giving and, of course, a visual use. To keep the wood visible, but still stiffen the hull, a layer of GRP is applied to the outside and inside. GRP is the short form for "glass reinforced plastic", i.e. a plastic layer reinforced with glass fibers. The glass fibers are applied as a fabric in twill or plain weave, whereby the twill weave is preferred in canoe construction because it adapts better to contours. The plastic is applied in the form of a two-component epoxy resin which, after mixing of the components, is very thin and thus settles around the glass fibers during impregnation. The refraction of light on the previously white-colored glass fibers is thus almost completely eliminated and the glass fabric "disappears" in the epoxy resin. The GRP accounts - in our estimation - for about 80% of the static of a canoe hull, the wood only 20%.
Before working with epoxy resins, be sure to read our article on health and safety with epoxy resins, as the material is not without danger!
What is so difficult about it?
Anyone should be able to laminate a canoe - but it's not easy to apply the laminate very clearly, evenly and free of unevenness. Our tips are intended to help you create a clear, even fiberglass laminate, because nothing is more annoying after the tedious process of planking than the look of a poorly applied fiberglass laminate.
Sanding the wooden hull
Before you apply the fiberglass, you should sand the hull properly until no more spots of glue are visible and all irregularities have been removed. Do a fine sanding in the direction of the wood grain with 120 or 180 grit - not too fine, so that the epoxy resin can bond better with the wood. Now wipe the entire hull once with a damp cloth so that the wood fibers stand up, let it dry and sand the raised fibers once again. Before you put on the fabric in the next step, you should very conscientiously remove dust from the hull and, if possible, also from your workshop (especially the floor).
Lay up the glass fabric
Buy the glass fabric rolled if possible, as folds can make laminating difficult and the folds are sometimes still slightly visible on the finished boat. You should handle the glass fabric like a raw egg: avoid stains from grease, otherwise the epoxy resin will no longer bond with the glass fibers and you will see the glassfibers in the laminate later. Also be careful to keep other contaminants away from the fabric. Wear cotton gloves when handling the fabric. Roll out the fabric, preferably with a helper, from one boat tip over the hull and gently smooth it out. Note that fibers shift easily, especially with twill weave. A clean, soft brush works well for smoothing.
Preparation of the epoxy resin
Choose a resin with a long processing time for lamination. In order to always work with fresh epoxy resin, we recommend that you mix several small quantities as needed rather than one too large quantity. To do this, weigh the components separately in 200ml plastic cups so that you only have to mix them when needed. Unmixed parts of resin or hardener can be tipped back into the containers later. Use an accurate scale to weigh the resin and hardener to maintain the mixing ratio. Calculate the required proportions of resin and hardener in advance according to the manufacturer's instructions. Prepare enough resin and hardener in case you need more. It is best to have a second person to help you mix while you laminate.
Laminating
When laminating, make sure the room temperature is warm enough, as this allows the resin to be thinner and soak deeper into the wood, creating a better bond. Start at about 25°C and lower the room temperature slightly during processing, as the wood can outgas as the room temperature rises and small air bubbles form under the epoxy that you can no longer remove.
Mix the resin components thoroughly by pouring back and forth between the cups several times to maintain the mixing ratio. Now mix the components with a wooden spatula until the resin is clear and free of streaks. Stroke the spatula along the sides of the cup and the bottom as well. Once mixed, time is of running as the curing of the resin is an exothermic reaction that releases heat. The longer the resin remains compact in the cup, the more heat is released and the faster it cures. So apply the resin quickly to the hull and spread it with a paint brush. Don't apply it too thinly so there is enough resin to soak into the wood. Otherwise, you run the risk of not saturating the fabric sufficiently.
Work wet-on-wet to maintain a wet edge, otherwise you may end up with visible transitions later. Again, it is an advantage to have a second person help you, as this way everyone can laminate one half of the hull.
Tip: do not use paint rollers to apply the first layer of epoxy resin, as these can bring air bubbles under the fabric that you cannot remove later!
Remove excess resin
Now allow the resin to soak into the wood for a certain time. How long this takes depends on the hardener and the specified processing time. At best, general statements can be made here, but a waiting time of approx. 10-15 minutes is realistic.
In the next step, you need to remove the excess resin so that the fabric adheres to the wooden hull. If you use too much resin, the fabric may float in the resin, which is often the cause of unevenness. Removing the excess resin is done with a plastic blade. There are plastic blades specifically for this purpose with a hardness that is matched to laminating epoxy resins. You will probably need a second person to remove it, as you will still be laminating after 10-15 minutes. Run the plastic blade from the keel down to one side in a fluid motion. Be sure to always keep the blade at a certain angle to the hull and always use the same pressure. The angle of the blade should be about 45°. If you hold it too flat, it will press the resin out from behind the blade, if you hold it too steep, too much resin will be removed. Always work with little pressure so as not to displace the glass fabric and not to press out too much resin. Wipe off the excess resin on the blade into a plastic cup and place it in a safe place. The resin in the cup can get very hot and foam up as it cures. Don't fill the cup too full and don't touch it again until the cure is complete. You must throw away any resin that is removed, it cannot be used again.
Photo: A kayak hull after removing the excess epoxy - the fabric structure is still visible. The fabric was cut at the stems and placed around each side to the other side of the hull. Thus, there is a double layer of glass on the stem, which often collides with stones. The edges parallel to the stem, visible in the photo, are leveled by the fillcoats and are no longer visible after sanding.
Fillcoats
After removing the excess resin, the wooden hull is visible again, but the fabric structure is still apparent. The first layer of resin must now be partially cured before several thin fillcoats are applied, forming an even layer over the fabric so that it disappears completely under the resin. It is also important to create a certain thickness of resin over the fabric, as you will be sanding the hull again later in preparation for varnishing. If you don't have the necessary safety in resin thickness, you may end up sanding into the glass fabric: you will never be able to touch up these areas, they will remain visible forever as white spots under the varnish.
The time to apply the first fillcoat is when the previous layer of resin is only slightly tacky, but no longer "wet". Do not check this with your skin, but with a nitrile glove!
You can also apply the fillcoats with a solvent-suitable paint roller. The rule is: it is better to apply more thin fillcoats than a few thick ones! You may also have to set an alarm clock in the middle of the night to apply the fillcoats ;)
Further tips
You should have the possibility to ventilate your working area, but note that insects are attracted by the vapors of the resin and like to fly into the sticky laminate. So equip your windows with fly screens.
Varnished hull of our Freedom15 canoe:
