Epoxy Resins and Boat Building: Epoxy as Adhesive
Generally, all boat building epoxy resins used for coating and fiberglassing can be used as an adhesive to bond porous materials such as soſtwoods, leather, un- glazed ceramics, etc. Tese epoxy systems work simply because they are stronger than the materials being bonded. Issues arise, however, when hardwoods, metals, plastics and the like are being bonded— especially when the service temperature is very low or highly elevated. Here, specialty epoxy adhesives designed for the specific application should be used. Regardless, one cardinal rule applies
when using any epoxy as an adhesive: Do not use excessive clamping pressure. Doing this with porous materials will result in a starved joint as any epoxy not mechanically squeezed out will be ab- sorbed into the pores resulting in a weak bond. Just close the joint leaving enough epoxy to form the bond then immobilize it until the epoxy cures. While the epoxy may cure to handling
strength overnight, it will continue to cure over the next several days gaining strength in the process. Many “glue fail- ures” result from stressing a joint before the epoxy is fully cured. Tis failure can be subtle: A tiller made from bent lam- inations may “unzip” over a period of several hours if removed from the mold too soon. Tis is not an “epoxy failure,” it is an
“application failure.” You can’t go wrong with more “clamping” time. Epoxy used in boat building should
be thickened enough to prevent sag- ging and excessive squeeze out. Silica thickeners (Cab-O-Sil, et al) are most oſten used. Builders using coating resins should thicken the mixed epoxy to a soſt, non-sagging paste for use as an adhesive. Prior to thickening, brush some mixed
liquid on the surfaces to be joined and then spread the thickened epoxy onto ei- ther of the two surfaces. Close the joint, immobilizing it with clamps, screws, staples, etc. Some epoxy will be squeezed out of the joint, but tightening should end before all is squeezed out. Clean up the excess while it is still fresh. Masking tape can be used to keep the wood clean where the squeeze-out occurs, but make sure to remove the tape before the epoxy cures. Epoxy will not bond well to the shiny side of packaging tape, so this can be used where appropriate to prevent the bonded pieces from sticking to a table
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Photo courtesy Marty Loken
top or similar substrate. Silica thickened epoxy will not cure
clear. If this is necessary for the job (e.g. woodwork) use a clear, thick epoxy like System Three T-88. Thick, clear ep- oxy adhesives are generally better than coating resins for bonding less porous hardwoods as well as aluminum and other metals. For more critical bonding use a spe-
cially prepared epoxy adhesive like our Silvertip GelMagic—available in car- tridges with mixer tips. Several boat ep- oxy companies offer cartridge-packaged adhesive products. Cartridges with tips eliminate user measuring and mixing, making a lot of applications quick, clean and simple. Clean soſtwood is ready to bond “as
is” without further preparation. Hard- woods should be rough-sanded across the grain and then cleaned with a solvent wipe. All solvent must evaporate before bonding. Aluminum is best etched with a phosphoric acid etch (Alumiprep, Alodine). All bonding surfaces must be clean of oil, grease, wax, etc., or the bond will fail. Some plastics bond well with epoxy while others don’t bond at all. Flame treating (search YouTube for a video on this) improves the bond on almost all plastics. Test epoxy adhesion by putting a little
dab of mixed epoxy onto the prepared surface and allowing it to cure. If it can’t be removed with a knife edge it is likely
well bonded. If possible test it at the service temperature it is likely to endure once placed in service. Always test if you are unsure. Epoxy-bonded wood should be
coated with epoxy to protect it from deep moisture cycling. Te epoxy coat- ing will stabilize the moisture content of the wood and prevent it from ex- panding or contracting enough to shear the glue joint. Cutting boards can’t be coated and so are never bonded with epoxy as the joints will open after a few weeks of use. Tis is not a problem when bonding metal to metal, plastic to metal, or metal to fiberglass as these materials don’t dimensionally change as the moisture content changes. Next issue we’ll discuss painting and
finishing. In the meantime, please email any
thoughts or questions to smallcraft@
systemthree.com and I’ll try to address them in future articles.•SCA•
Kern Hendricks Bio: BS in Chemical Engineering fom Case Western Reserve University. Developed an epoxy system that was used to build a Jim Brown/John Marples 34-foot Searunner trimaran started in 1975. Founded System Tree Resins with Tom Freeman in 1979. Built a kit airplane in 1995 and still flies it today. Now semi-retired and touring the U.S. in his motorhome with his wife. Lives in the Seattle area.
SMALL CRAFT ADVISOR
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