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. They are structural adhesives and can hold the neck of a guitar against the tension of the strings. These characteristics make them ideal for gluing and repairing wood and other materials. Here we show you how to do this to maximise the strength of your repairs and joints.
This article details the important things you need to know to glue any wood. Details of how to glue metals can be found here and how to glue plastics is found here. You can also learn how to repair Cracks and Splits in old gunstocks.
Application of the Epoxy Wood Glue on the Surfaces to be Glued
Avoid Glue Starvation
Apply any liquid epoxy adhesive to both surfaces to be glued for best results. Then allow it to sit for long enough for the wood to soak up as much as it wants to. The assembled joint will not then further absorb the glue that should fill the gap between the pieces. That soaking leads to a glue-starved joint. Scarf and butt joints are especially prone to soaking glue out of the joint. The end grain wicks up the liquid glue. End grain 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. You have to forcefully smear glues like this against the entire surface to be glued. Otherwise the paste or gel glue never actually wets the surface of the wood. A low-strength glue joint results otherwise.
Even that won’t assure a strong glue joint, as penetration into the timber will be very shallow.
Acidic or Oily Timbers
Many woods have acids [such as oak] or oils [such as teak] in them. Most epoxy products do not cure in contact with acidic woods, or cannot wet and stick to oily woods. Click here if you want to know why our Oak and Teak epoxy resin glues these oily and acidic 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 forces pulling them apart. The same forces applied by the clamp to close the joint in fact. Possibly as much as a metal C-clamp can develop. However, 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. This happens within hours to days after the clamps are removed. Wood splits easily. That, basically, is what is happening here.
The best way to make curved beams is by steam-bending the individual laminations. Drying them in a fixture sets the new shape. They are then glued together once they have adopted their new shape. Steam-bent wood holds its new shape without stress. Boat ribs have been manufactured by steam bending 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 will 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. The natural porosity of wood wicks away the remaining microscopic residue of glue. The result is a glue-starved joint.
This often fails 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
Clamp the wood element 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 well. This is because the acids, saps and resins in the wood interfere with the bonding chemistry of the adhesive. We specially formulate our own glues (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 epoxies are further compatible with the acids which occur naturally in many woods, particularly oak. Thus, our epoxy adhesives may directly glue hardwoods such as maple, acidic woods such as oak, and oily woods such as teak, apetong, araki, pau lope (Ipe), Osage orange, etc. For a full list of compatible timbers see here.
Smith and Co. make many opoxies, of which Oak & Teak Epoxy Glue is only one. Check their website for further details. That said, if you are in Europe, you will be buying from us, so it’s Oak and Teak Epoxy.
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. Clean this material off the surface before gluing by wet sanding or light abrasive blasting (such as glass bead or 200 mesh abrasive). This is effective in improving the bond strength of these joints. Side grain bond strength however, even with ebony, is 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 wood will absorb the solvents 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 before the glue has cured can, in some cases, cause 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, those that do not bond well to oily hardwoods, invented the idea of solvent-cleaning the wood. Hoping this will make their products stick to the wood. That is the origin of this idea. The problem is due to their glues incompatibility with the natural oils of wood,
Because the solvents become absorbed into the wood this is a bad idea. It takes a LONG time for these solvents to diffuse out and evaporate. The glue then absorbs these solvents. Thus, as the glue cures, the high solvent content right where the glue contacts the wood weakens it. 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.
You can directly glue woods including 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
When mixing two-component products, it is important to thoroughly mix the glue 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
You must adopt certain safety procedures with all modern products 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
Worlds Best Epoxy Wood Glue?
Author’s note (Stephen Dakin): I genuinely believe, and I have spent over 40 years gluing things together, that this is the best epoxy resin that I have ever used. It’s performance is quite literally amazing. If you ever work with wood, buy some and try it, you will end up using it for every instance where joint strength really matters.
© copyright Steve Smith and Stephen Dakin, 1972 – 2015, All Rights Reserved.