Laminating - getting glass on the wood hull, wet-lay up overview
First-time canoe builders in particular get excited by the woodworking, but suffer panic attacks when it comes time to laminate their canoe. Cedar strip canoes actually have more in common with contemporary plastic canoes than yesteryear’s wooden hulls. Laminating, the task of applying epoxy and fabric reinforcements is a dramatic milestone. Success lies with mindful perations. Plan to invest 20 hours in sanding and preperations in each side of the hull, only two hours for the actual wet layup. A single cure cycle maximizes lamination strength and minimizes sanding. The process flows like...
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Fair and Sand the Hull
By now, you've eradicated staples, scraped off glue globs and faired the hull. The next step is to sand to 60/80 grit. Stop. Pay no attention to the little woodworker gnome whispering in your ear to sand all the way to 220-grit. It's a canoe not a piece of furniture. A slightly textured surface provides a stronger bond between wood and laminate skin.
The shoulder gnome may also insist you fill staple holes with wood dough. Avoid using oil base wood fillers if you follow his advice. If staple holes bother you, brush boiling water or use a steamer to raise the grain and shrink holes. Follow with a light sanding. Thickened epoxy fills staple holes during wet-out, eliminating a cure / sand cycle. Some canoe builders (we are in this camp) find staple holes no more offensive than spruce root, clinched-over copper nails or rivets. We accept fasteners on nearly every other type of marine vessel, why the stink over staple holes among cedar strip canoe builders.
Get under the canoe and look up. If you see light apply masking tape. Minor voids fill nicely with epoxy during lamination. Taping prevents epoxy from draining through, which can result in dry spots (a white resin starved area) in your exterior lamination. If you prefer to fill gaps before the wet process, laminating resin thickened with Cabosil and a pinch of sanding flour to match color makes great filler. It takes about a tablespoon of thickening agent(s) per ounce of resin to get a to the ideal creamy p-nut butter viscosity. If you gap fill at the sanding stage rather than during the wet process, allow time for full hardness and sand to clean wood with 60-grit.
Vacuum the canoe to rid of dust. Avoid using a tack cloth, solvents or thinners on bare wood. Now you are ready to dry fit your fiberglass.
Dry Fit Fiberglass
For a recreational canoe you typically use 60-inch wide 6-ounce fiberglass. In most cases you need to purchase lineal yardage slightley more than double the length of the canoe. That's enough to cover bias strips to reinforce the stems or a few strategically placed interior ribs. A smaller solo hull may get by with lighter 4-ouce fiberglass. A super duty tripping canoe may benefit form 8-ounce cloth or multiple layers of lighter cloth.
The primary exterior piece needs to extend a few inches beyond either end of the canoe. Just like strips, a 16-ft piece of cloth will fall short on a 16-foot canoe because of the arc. A chubby fishing vessel will take more cloth than a sleek solo of the sale length.
Cut one, two or more bias strips for each stem. The additional layers add abrasion protection where you need it most. To cut on the bias, fold the width to the selvage edge (Make a triangle). Use the new angled edge for the long side of your bias strip. Cut both layers at once and you'll have matching bias strips, one for each stem. Make two or three progressively sized strips. Apply the smaller strips first when you laminate; this eliminates edges that require sanding later.
Avoid the temptation to add a full second layer of fiberglass to the exterior unless you are building a grand laker size canoe or paddling extreme conditions. More exterior layers allow more sacrifices to river rocks, but add a lot of weight with little benefit for the recreational paddler.
Envision the canoe in cross-section. Looks like and arch doesn't it? Since Roman times engineers have understood, making the arch stronger happens by strengthening the interior curve. The same principle holds for the canoe. If you want to build a stronger canoe, beef up the interior lamination. You can maximize strength; minimize weight gain with interior pieces on the bias and below the waterline.
Feeling adventerous? There's no law that you must use a clear fiberglass laminate on a cedar strip canoe. Kevlar and carbon fabric reinforcements work just as well on wood as over foam core.
Calculate Amount of Resin
Canoes gain weight when canoe builders apply more is better logic to laminating. Monkey with the 1:1 glass to epoxy ratio and the canoe does not get stronger, it just gets heavier. Simple calculations and planning ensure a stronger lighter canoe.
Start by confirming the surface area of the hull. You do not need a nautical engineering degree sophomore geometry will suffice. Use a flexible dressmakers tape to measure the working edge of each building form. Add the length of two adjacent forms, divide by two and multiply by the station spacing. You just calculated the area of a trapezoid in square inches. Divide by 1296 for square yards; add the square yardage together for total surface area of your canoe.
Fiberglass absorbs one ounce of resin by weight; hence, six-ounces of epoxy will wet out one square yard of six-ounce fiberglass. A typical tandem canoe - eight square yards of surface area will require 48 ounces of catalyzed epoxy to wet out the fiberglass. Naked cedar drinks up three-ounces of epoxy per square yard. That brings your estimate to 72-ounces. In addition, plan eight-ounces of Cabosil thickened epoxy to fill minor voids (give or take - depends on how tight your hull is and whether you build with or without staples).
If you come up short, large areas of unsaturated fiberglass, look at the floor. If the floor is clean, fiberglass is floating on a pool of resin somewhere on the canoe. Look for shinny areas; use firm squeegee pressure to redistribute the excess epoxy.
Stage Tools and Supplies
Roll fiberglass or make accordion folds. Rehearse folding and unfolding or rolling and re-rolling until you feel you can replicate, placing the cloth right where you want it on the first attempt. Common practice at our shop is to lay a double roll - Dead Sea Scroll style - on the center of the canoe. If your lamination schedule calls for multiple pieces of fabric, sequence the parts in reverse order for easy access as you apply each part to the hull. Lay out spreaders, rollers, brushes, gloves, scissors, X-acto knife and any other tools you expect to use.
Stage resin in individual cups and mark each cup with the number of ounces / pumps. The amount of time you just spent measuring resin adds to your wet working time. Now you only have to pour / pump hardener once the hull is wet. Working alone you may only catalyze 10-ounces at a time, with a crew; twice that amount gets spread out very quickly.
The AdTech ProBuild and ProClear epoxy laminating systems distributed by NorthWest Canoe are premium marine epoxies, widely used in commercial marine and aerospace and now available to the hobby builder. Both epoxy systems provide UV stabilized low viscosity epoxies. ProBuild offers four hardener options. Most canoe builders find medium or slow works best for them. If you live where the temperature does not drop below 85 degrees for months at a time, consider tropical hardener.
Once opened, epoxies have a one-year shelf life. Mix a test batch of any leftovers before you laminate your canoe to make sure it catalyzes/hardens properly. Do not mix resins and hardeners from different manufacturers together
At NorthWest Canoe, we use a spreader/squeegee and lint-free mohair roller. With squeegees you move resin around quickly, control pressure and avoid stray brush bristles and minimize bubbles. Some canoe builders recommend brushes. The tool you settle on comes down who's book or website you've read and what you feel most comfortable using. They all work.
The Seal Coat
Wet the bare wood with epoxy. Again, naked cedar absorbs about three ounces of epoxy per square yard. When hand laminating, wetting the wood before laying the fiberglasss achieves a more thorough (clearer) laminate. That's because the wood is not wicking resin off the backside of the fiberglass.
Catalyze (add hardener) to the first staged cup of resin and mix according to the manufactures directions. Pour the catalyzed epoxy down the keel line and begin working with roller or spreader towards the sheer line and the stem. Spreaders are less likely to introduce air bubbles and work well to cut what you need from the pool. Pull diagonally, towards the sheer line and the stem. It takes fifteen minutes to apply the seal coat.
You can stop here if you want. If you choose to not lay the fiberglass into wet epoxy, you will need to sand the hull once the seal coat cures. fully cured polymers do not cross link; you only achieve a mechanical bond. Remember at the onset we talked about a stronger lamination? Envision a block of limstone. If you laminate your canoe in stages you create strata similar to limestone. Continue working wet all the way through to the fill coat for a stronger fully cross-linked laminate, think granite.
Immediately after the seal coat, catalyze a cup of thickened epoxy (use fumed silica, micro spheres, etc.) to fill staple holes, small voids. There's no formula for how much Cab-o-Sil (silica) to add, go for that rich creamy cake frosting consistency. Thickened epoxy will flash off quickly if you leave it in the mixing cup; so walk around the canoe and place dollops where needed, then come back with a spreader to work into holes and cracks. Once you hit the obvious spots, work the entire hull in a diagonal cross hatch pattern to even out and wipe off excess thickened goo. This entire process should take no more than twenty minutes.
Appling thickened epoxy during the wet process helps prevent resin-starved spots. It also eliminates the need to fill, scrape and sand before laminating, reducing the risk of accidentally sizing and staining the wood with glue based wood fillers.
Wet Out Fiberglass
Make laminating an orderly process that starts when you pour out the first cup of resin. yes, pour out. Unlike paint or varnish that dries as carriers evaporate leaving behind the small percentage of solids, epoxy is 100% solids that cure via chemical reaction when you combine the A and B sides. That exothermic reaction becomes more intensse as mass increases. In other words, don't allow catalized epoxy to sit in your mixing container like paint. Walk from bow to stern and pour whatever size container of resin you catalyze along the relatively flat keel line of the canoe and work it with your suegee. Alternatively, pour the cup onto a flat container like a cookie sheet and work with your brush or roller from there.
Visual the hull in quarters. Work each quarter separately, pulling resin away from the center toward the stem and sheer. This pattern ensures you work the fiberglass on the biais. In our experience, fabric wets out more quickly working on the 45 rather than in the direction of warp/weave fibers. As the hull transitions from horizontal to vertical outsmart gravity by rolling your wrist/squeegee face up to hold the epoxy. If you're still plopping epoxy on the floor when you transition to vertical try taking smaller bites from the resin pool with your squeegee or swith to a roller. Use a light touch and low squeegee angle to move resin onto still white unsaturated fiberglass or wrinkled areas, this avoids snagging the fabric. Ingnore wrinkles untill every square inch of fiberglass turns clear.
Lay a rectangular two-dimensional piece of fabric over a curvy three dimensional canoe hull and you can not help but introduce a few wrinkles. The first thing you need to do is actually a do not... don't panic. Second, don't spend your time fussing over a single wrinkle. Third, don't reach for the razor blade and slice the wrinkle open.
Handle the situation like this: continue to wet out the fabric. That's right, catalyze that second cup of resin, pour it down the keel line of the canoe. Go to the center, grab a bite of resin from the pool laying along the keel line and pull towards the sheer and the stem. Use a light touch as you pull over dry cloth to avoid snagging the fabric.
Pulling toward the sheer line and the stem, you're working the cloth on the bias. Working this diagonal pattern you will naturally pull more than half the wrinkles out. As for for those stubborn wrinkles that won't go away, with the cloth fully wetted out (fiberglass completely clear), work the cloth with the flat of your hand. With wet fabric you can lay palms on opposite sides of the fabric, pull gently on the bias and watch an entire family of wrinkles vanish.
Trim Excess Cloth at Stems / Ends
Sixty-inch wide cloth fits gunwale to gunwale around the belly of a tandem canoe just right. By the time you get to the stems, you have excess cloth. In this snippet, we share how to deal with the wet excess and sticky fiberglass that prefers an unravel state.
You will need a stack of heavyweight paper and razor sharp knife. The paper stock serves as a backing to make your cuts. We use the same letter-size 110lb index we feed the office printer on occasion. Omit the paper and you will make a dark score line on the wood hull. A curved blade works far better than a sharp pointy knife. Points tend to randomly snag individual yarns and make a mess of things. My personal favorite tool for the task is the Flexcut Pelican Knife.
Wait until the surface feels like flypaper to fill the weave, about a two-three hour break. If you can still move the cloth it's too soon. Spreaders definitely work best to push resin into the little pockets between warp and weave fibers.