How to Repair Wood With Epoxy
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Soft woods such as fir and pine can absorb liquid epoxy adhesives, especially in butt or scarf joints where end grain is exposed. Thickened (jelled or mineral filled paste) epoxy adhesives glue only onto the surface, and the surface of wood consists microscopically of broken cellulose fibers (which are hollow thin ­wall tubes). Contact only on the surface does not produce as high-strength a joint as contact by an adhesive that flows well and wets the microscopically rough surface instead of bonding to the high points only. Softer woods have very low shear strength and so an epoxy that soaks in and impregnates the wood offers a higher strength bond as it glues the wood surface down into the bulk of the wood. This allows a thick liquid or paste filler to develop a strong bond with the wood itself. Our Clear Penetrating Epoxy Sealer was especially developed as a sealer for soft woods such as fir or pine. It also serves as an adhesion-enhancing primer on any wood before applying oil-base (alkyd) varnish, clear two-component polyurethane finishes, or enamel or even latex paints. Harder woods such as oak, teak, maple, alder, apetong, araki, pau lope, osage orange, etc., may be glued directly with our Tropical Hardwood Epoxy or Oak and Teak Glue, as they are dense enough that adequate surface area is available for a high-strength bond. Any of our epoxy adhesives may be used to glue soft woods. 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, leading 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 end of the hollow cellulose tubes of which the wood is made. Edges of plywood are notorious for soaking up liquids. Most adhesives, even epoxy adhesives, do not bond hardwoods because the saps and resins in the wood interfere with the bonding chemistry of the adhesive. Our Tropical Hardwood Epoxy or Oak and Teak Glue is specially formulated (by us - we're chemists) 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. 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. ISide 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. Both the Oak and Teak Glue and the Layup & Laminating Epoxy are high-­strength adhesives, but not as flexible as the Tropical Hardwood Epoxy, which is excellent for dissimilar woods and cross-grain joints due to its ability to absorb stress. Our Fill-It epoxy filler is fine-consistency wood filler for filling rough surface tool marks or for major restoration work. It is not intended as thin-film glue. In order to minimize sanding, a sheet of clear polyethylene may be placed over the wet Fill-it and a squeegee used to remove excess material. When the epoxy cures, the polyethylene film is released and may be removed, and a smooth surface is obtained. 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 jells. 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 also to a greater or lesser degree 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. Epoxy products develop extremely high-strength bonds to wood, much higher strength than polyester resin products (the ones that take one or two percent of a "catalyst"). With all modern products there are certain safety procedures that must be observed if the user is to avoid a rash or allergy developing. Do not get the 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.