Featured Woodworker Editorial

Which Glue?

By Ken Horner

How do you know which glue to use?  When your grandfather opened his shop in the morning, he started a fire in the pot-bellied stove and then set the glue pot on top to start heating. No matter what the project was or what kind of wood, he used hide glue.  Today we get to (must) choose from nine different types of adhesives with multiple choices within the categories. 1. Hot Melt – These are cheap, easy to apply and set up instantly. They are used mostly for models, mock-ups, one-time jigs and temporary attachments. 2. Contact – These are made from synthetic rubber and are applied to both surfaces and allowed to dry before mating. The cement bonds without clamping and the joint is heat- and water-resistant. They bond plastics, veneer, rubber, metal and canvas. In the shop, this adhesive is used in veneering, marquetry, inlay and whenever dissimilar substances are to be joined. 3. Cyanoacrylate (CA) – These are quite expensive but will bond most common plastics, hard rubber, glass, hard PVC, glass, metal, ceramics and skin; in fact, doctors use it to close wounds instead of suturing.  CA sets up in 5-10 seconds and has great tensile strength. They have good water and solvent resistance.  4. Epoxy – These are expensive but bond almost anything to anything. They’re sold as two-part, cold liquid mixes (resin and hardener) that are mixed just before use. They are good at gap filling, are strong, water proof and need no clamping. 5. Polyvinyl Acetate (PVA) – These are the common yellow and white carpenter’s glues we all use in our shops. They are water-based, have no odor and are nonflammable; they cure by water evaporation. The white PVA has no water resistance; the open assembly time is about 15 minutes, and the glue can be reactivated by heat (180ºF.) indefinitely. Marquetarians and veneer people love this glue. The yellow variety has better sand-ability and develops tack faster than the white. It is less prone to creep and has better water resistance.  It can be reactivated by heat (250ºF.) for about 10 days. The cross-linking variety is water proof, has better initial tack and can be reactivated (350ºF.) for a few days. 6. Urea Formaldehyde – These are sold as a powder to be mixed with water. They are very strong and have no creep.  They are used to make plywood and laminates. I used this glue on my red oak dining table top.  7. Resorcinol – This glue is completely waterproof; it was developed during WWII to assemble wooden boats. A liquid resin is mixed with a powdered hardener; open assembly time is 30 minutes, and clamp time is one hour. This glue is great for outdoor projects. 8. Polyurethane – These glues are spread on one surface only and react with moisture in the wood. They work well on moist, oily woods but have no gap-filling strength.  The open assembly time is about 20 minutes, and the joint sets in 60 minutes. They are nontoxic, waterproof and can be used with metal, plastic, ceramics and stone. 9. Natural Protein – These glues are made from horse hides, rabbit and fish skins, rice, milk and blood. In all cases, the active ingredients are proteins. Furniture and musical instrument repair sometimes dictate the use of these glues, but in large part they have been replaced by the PVA glues.