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How to Glue and Repair Wood with Epoxy - Smith & Co.


The important things you need to know to glue any wood

Application of the adhesive on the surfaces to be glued

For best results any liquid epoxy adhesive should be applied to both surfaces to be glued and allowed to sit long enough for the wood to soak up as much as it wants, so that when the pieces are assembled the wood will not absorb the glue that would otherwise fill the gap between the pieces. That leads to a glue-starved joint. Scarf and butt joints are especially prone to soaking glue out of the joint, as it wicks into the end grain of the wood, which is the open ends of the hollow cellulose tubes of which the wood is made. Edges of plywood are notorious for soaking up liquids.

Improper clamping of wood may be the largest source of glue-joint failure.

Poorly-fitting wood elements, clamped to bend them into contact, will have tremendous spring-back forces pulling them apart, as much as a metal C-clamp can develop. Yet, the shear strength of wood is only 200-300 pounds per square inch. The result is that the wood fibers at the glue joint will tear away from their parallel neighbors, often within hours to days after the clamps are removed. Wood splits easily. That, basically, is what is happening here.

Curved beams are best made by steam-bending the individual laminations, letting them dry in a fixture that sets the new shape, and then gluing them. Steam-bend wood holds its new shape without stress. Steam-bent ribs for boats have been used for at least hundreds and perhaps thousands of years. If steam bending is not an option, cross-grain fasteners or splines (tenons, biscuits) should be screwed or glued at each end, because the curved structure will want to straighten, and the glue joint will fail by cleavage.

Straight, smooth, well-fitting wood elements can yet be made to fail by using excessive clamping force. Squeezing a glue joint down to zero glue-line thickness forces out almost all the glue from between the pieces, and the natural porosity of wood wicks away the remaining microscopic residue of glue. The result is a glue-starved joint. It will often fail when the clamps are removed, or sometimes days or weeks later. If the failed joint shows no divots of wood pulled out of the opposite side, the cause of the failure is almost certainly excessive clamping pressure and not enough time allowed before clamping. The wood element should be clamped gently: Just enough to squeeze out the excess glue and bring the wood pieces into contact at the microscopic high points of the joint. Thick pads of soft rubber under the clamp faces ensure gentle, even clamping forces.

Woods may have acids or oils; some glues do not like that.

Most adhesives, even epoxy adhesives, do not bond hardwoods because the acids, saps and resins in the wood interfere with the bonding chemistry of the adhesive. Our glues are specially formulated (by us - we're chemists here at Smith & Co.) to overcome this difficulty. We designed a chemical system that would absorb and displace the saps and resins without becoming weakened by the absorbed oils. Our formulations are also compatible with the acids naturally found in many woods, particularly oak. Thus, hardwoods such as maple, acidic woods such as oak, and oily woods such as teak, apetong, araki, pau lope (Ipe), osage orange, etc., may be glued directly with our epoxy adhesives.

Oak & Teak Epoxy Glue is only one of several epoxy adhesives we make. Call the Factory Store at 1-800-234-0330 to discuss your particular requirements.

Some woods, particularly ebony, contain a wax rather than oils. Saw cutting or dry sanding can smear this wax over the surface, making gluing difficult, especially on end grain or 45 degree bevels. Wet sanding or light abrasive blasting (such as glass bead or 200 mesh abrasive) can clean such material off the surface to be glued and has been found effective in improving the bond strength of such joints. Side grain bond strength, even with ebony, was found adequate with saw cut or dry sanded surfaces.

It is important to remember that wood is a natural product and varies.

It is also important to remember that surface preparation is at least 50% of adhesive bonding technology.

Our products have fairly long thin-film set times, and so the user has plenty of time to wipe up drips or shape into the desired form before the epoxy gels.

Do not use solvents to "clean" hardwoods before gluing.

The solvents are absorbed by the wood and will cause the epoxy bond to the wood to fail. Even solvent cleaning hardwoods after gluing (while the glue is still wet) has in some cases, caused glue-line failures. Wiping up drips with paper towels is safe. These comments apply not only to our glues, but to any glue on any wood.

In mixing two-component products, it is important that the product be thoroughly mixed or it will be physically weak when cured. One of the most dependable methods of ensuring complete mixing of liquids is to mix well in one container, transfer to a second container and mix again.

With all modern products there are certain safety procedures that must be observed if the user is to avoid developing a rash or allergy. Do not get epoxy or other resins on your bare skin. If you do, stop what you're doing and go wash with soap and water. While casual exposure at infrequent intervals may not be harmful to most people, it is impossible to predict who will become allergic after some exposure. So, be neat and work clean.

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