Epoxy resins have some wonderful attributes that are not shared by all adhesives. They can fill gaps, and can normally be sanded easily to leave a pretty glue line. These characteristics make them ideal for gluing and repairing wood and other materials.
This article details the important things you need to know to glue any wood. Details of how to glue metals can be found here.
Application of the Epoxy Wood Glue on the Surfaces to be Glued
Avoid Glue Starvation
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.
Many glue manufacturers have “solved” this problem by making epoxy glues that are gels or pastes, not liquids. Unless the user smears a thin film of such a glue forcefully against the entire surface to be glued, the paste or gel glue never actually wets the surface, and a low-strength glue joint often results. That alone is not enough to assure a strong glue joint.
Acidic or Oily Timbers
Many woods have acids [such as oak] or oils [such as teak] in them, and many kinds of epoxy products do not cure in contact with acidic woods, or cannot wet and stick to oily woods. More about this later, but that’s not the first and largest reason for failure of wood glue-joints.
Clamping the Pieces to Ensure a Strong Glued Joint
Ensure the pieces fit properly
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 fibres at the glue joint will tear away from their parallel neighbours, 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-bent 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.
Don’t Squeeze Your Glue Out
Straight, smooth, well-fitting wood elements can 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/or not enough time allowed after glue application, for the glue to soak into the wood, 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. Remember, epoxy wood glue is very good at filling gaps in joints.
Woods may have acids, oils or waxes; 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. For a full list of compatible timbers see here.
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.
Solvent-cleaning wood before or after gluing
Do not use solvents to “clean” oily hardwoods before gluing. The solvents are absorbed by the wood and will cause the epoxy bond to the wood to fail, or to be far weaker than it would otherwise be.
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.
Who said ‘clean with solvents’ before gluing oily wood?
Some manufacturers of glues that do not bond well to oily hardwoods, due to their incompatibility with the natural oils of wood, invented the idea of solvent-cleaning the wood so their products would stick to the wood. That is the origin of this idea. It is a bad idea, because solvents are absorbed into the wood, and take a LONG time to diffuse out and evaporate. Those solvents will be absorbed by the glue itself, and when the glue cures, it will be weakened by the high solvent content right where the glue contacts the wood. In effect, much of the glue is bonding to solvents instead of to the wood, and such glue-joints come back apart rather easily. That’s why solvent-cleaning wood or applying solvent-containing products to good-quality wood that has any reasonable amount of strength is a bad idea.
So what do I do for Oily Woods
Our recommendation, find a glue that works with the oils in the wood, and can create a really firm glue joint without requiring solvent cleaning. Smiths Oak And Teak Epoxy Wood Glue will glue all of these timbers without problems.
What about Using Epoxy Wood Glue On Very Weak Timbers?
Epoxy Wood Glues are by their very nature enormously strong, but any bond will be only as strong as it’s weakest link. If the timber species itself is very weak, it may require consolidation first to give a surface strong enough to support the joint.
Woods that may be directly glued include oak, teak, old-growth redwood [typically 20-50 growth rings per inch] and similar woods. Woods that require consolidation of the fibres before they are glued include “second-growth redwood” a physically weak and very porous wood, with perhaps 4-8 growth rings per inch. In that case Clear Penetrating Epoxy Sealer™ has proven helpful, particularly when glued with the very flexible Tropical Hardwood Epoxy™.
Mixing two-component Epoxy Wood Glue Products
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.
Hygiene and Safety Working with Epoxy Wood Glue Products
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; always wear gloves and appropriate personal protective equipment.
Final reminders for gluing any wood with any glue
- 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
- If you use products that have fairly long thin-film set times, then you will have plenty of time to wipe up drips or shape into the desired form before the epoxy gels
© copyright Steve Smith, 1972 – 2015, All Rights Reserved.