|Wood Fasteners, Adhesive|
Household glues having low moisture resistance have tendencies towards early joint failure and should be avoided in marine applications.
Resorcinol and Phenol-Resorcinol resin type marine glues have been used for many years and are satisfactory for most new construction and repair applications. Resorcinol age hardens and becomes brittle and inelastic over time and should be limited to rigid surfaces where shear, vibration and impact forces are unlikely.
Urea-type adhesives such as Weldwood Plastic Resin glue are available in water mix one-part and two-part resin/hardener mixes. Use of ureas requires special care particularly with the two part system as, unlike epoxy resins, the urea is applied with resin on one surface and the hardener on the other. Clamp pressure is then applied and the cure begins.
Epoxy resins are available for a wide variety of marine applications and have been found to provide excellent adhesion in all areas of boat building. In the early 1960's epoxy adhesives were introduced to western boat builders by the Gougeon Brothers of Bay City, Michigan, through their registered trademark WEST SYSTEM. Epoxy resins are two part adhesives and depend on accurate mixing ratios to yield high strength joints. Epoxy is also an excellent filler material when thickened to high or low density with micro fibers, micro balloons or colloidal silica
Not all woods are easily joined. Wet wood (above 18% moisture content) is difficult to glue. Normal seasoned wood of most species can be glued. Strong joints can be made bonding either face or side grain of the wood. These joints can be very nearly as strong as the wood itself. It is impossible to join end grain with glue and get joints which are even 20% as strong as the wood. A scarf or some other form of joint which gives a surface approaching side grain condition must be used where end connection is desired.
As with any chemicals the manufacturer's instructions must be carefully followed. Curing temperature and surface condition are important. The temperature must be about 70 degrees Fahrenheit or higher for a full cure of resorcinol resin glue. Faying surfaces should be well fitted. Smooth surfaces make the strongest joints with resorcinol, however a roughened surface for epoxy joints is generally helpful in improving bond strength, especially with hardwoods, such as oak.
Excerpted from Wood Hull Inspection Guidance (NVIC 7-95)