Glue ManagementComments (0)
This article is from Issue 42 of Woodcraft Magazine.
From project prep to post-construction
By Robert J. Settich
Just as a smart coach matches the strengths of individual players to the challenges of various situations, a shrewd woodworker selects glue based on its unique qualities and the project at hand. Here’s a game plan, including tips, to help you choose the right glue and for applying it only where needed.
Go-to glue lineup
Yellow glue is a generic term for a variety of woodworking adhesives, such as the Titebond brand manufactured by Franklin International. All three Titebond formulas clean up with water, but the key difference is the water-resistance of the dried glue film. Titebond Original, an aliphatic resin, has low water-resistance, but it works for a wide range of interior woodworking applications. Titebond II, a cross-linking polyvinyl acetate (PVA) formula, is a good choice for bath and kitchen cabinets and other projects expected to endure high-humidity environments. Titebond III, considered a waterproof glue, does well for outdoor furniture construction, though it’s not recommended for immersion in water.
Polyurethane glue, such as Gorilla Glue, features similar qualities as Titebond III, but with the glue applied to only one surface. Here, as recommended, dampen the contact surfaces with water before bonding. Polyurethane features an ample open time of 20 minutes, but it produces a foamy squeeze-out that mandates careful protection of adjacent surfaces. You can try removing the wet adhesive with denatured alcohol, but you may want to to wait, and then scrape away the cured residue. Take advantage of polyurethane’s high water resistance when building outdoor furniture.
Liquid hide glue provides all the advantages of this centuries-old animal adhesive in a ready-to-use formula. Its bond is reversible by the application of moisture, making it an ideal adhesive for musical instruments and other projects that may require disassembly. And unlike other adhesives, its dried film only minimally resists the penetration of stains and finishes. So it’s a good choice for goof-proof general assembly and even applying veneer. It also features a longer open assembly time than yellow glues, meaning that it doesn’t grab as quickly. Whereas yellow glues have an open time of 5 to 10 minutes, liquid hide gives you 20 to 30 minutes, an advantage when working with complex assemblies.
White glue, such as Elmer’s Glue-All, is a PVA with an open time of about 10 minutes. It’s a good choice for general assembly when water resistance isn’t critical, and a recent reformulation makes it stronger than ever. That said, don’t confuse white glue with Elmer’s School Glue, which is also white but not recommended for woodworking.
Cyanoacrylate (CA), or instant glue, comes in various viscosities to solve a wide range of problems. The thin version easily wicks into cracks to stabilize the wood or can bond non-porous materials such as metal to wood. These features make CA a favorite among woodturners and pen makers. Thicker formulations cling better to improve handling. CA is an excellent choice for assemblies that are difficult to clamp, such as grilles for windows. You apply the adhesive to one surface and spritz an activator (or accelerator) onto the mating part. This approach creates an almost instant set, allowing 3 to 8 seconds for joint adjustment, depending on the formula’s thickness. However, if you apply CA to the assembled joint and apply the activator after, you buy a few seconds of adjustment time. You double adjustment time when you defer using the activator. If working close in, wear gloves. The adhesive can aggressively grab flesh. Use acetone to de-bond fingers or clean up wet adhesive.
Epoxy takes many forms, from thick as putty to syrupy. Using it requires that you mix a resin with a catalyst to initiate a chemical reaction. Due to its high water resistance, epoxy and fiberglass are sometimes combined with wood to build boats. It’s also used to strengthen rotted or punky wood. Also, you can take advantage of epoxy’s unique gap-filling ability to fill splits and voids. Like CA glue, it too excels at bonding together wood and non-wood materials.
When Good Glue Goes Bad
In a tightly sealed container stored away from sunlight and extreme temperatures, yellow and white glues can last two years. If your shop isn’t climate controlled, store your glues in a heated space, such as your home. If the glue gels or becomes stringy, or if it turns color as seen at right, toss it–it’s strength is gone and the glue is beyond salvaging.
Related to shelf life lies the issue of chalking. On many glue bottles you’ll find a curious entry called “chalk point,” along with a temperature reading. This is a crucial number because glue that dries below this temperature will have a whitish, chalky, appearance. Even worse, that symptom indicates that the glue molecules have failed to knit together. For this reason, keep your shop 55° F or above during project glue-ups.
Some woodworkers believe that the best glue applicators are the five they have on each hand. But glue on fingers often finds its way onto project surfaces, showing up as light spots that refuse to accept stain. Besides, the low pH and chemical content of glues can cause skin irritation.
Disposable glue brushes, craft sticks, toothpicks, and wood scraps are well-known applicators, but there are many commercial spreaders that help you place glue exactly where you want it.
A glue roller helps you quickly spread glue over a large area, though you could also improvise with a plastic hotel room card. The biscuit slot applicator makes a controlled deposit into plate joinery slots, simplifying and speeding an otherwise tedious task. Glu-Bot eliminates the annoyance of shaking a container to start the flow. Its tapered nozzle helps deposit glue precisely.
Plastic spreaders and glue brushes are inexpensive and reusable. When you’re finished applying glue, simply wash the brush clean or let the glue dry on the spreader. You don’t even need to wipe off the excess. The glue–even dried epoxy–pops free.
Finally, injectors deliver glue into tight areas, helping solve problems such as chair-rung and separated veneer repairs.
Prep work for strong joints
A successful glue-up begins with a properly-fitting joint that has clean, freshly prepared surfaces. Although that may seem obvious, woodworkers often ignore this stage hoping modern glue chemistry and brute clamping force will rescue their project. I’ll tackle each phase of joint prep and tell how to maximize strength.
Great joints fit together without a need to beat on them or strain on a clamp handle. These situations indicate a joint under stress, and that pressure can crack the wood or twist the assembly. On the other hand, you want to avoid loose joints that rattle. Glues work best in a thin, even film between the two pieces of wood. If a void does exist in the joint, your options are to remake the joint or, though less desirable, use epoxy, letting it serve as a strong gap filler.
Mask gluing surfaces to prevent the stain from compromising the joint.
Inspect machined joints for any evidence of burning or glazing. These conditions impact a glue’s ability to penetrate the surface and bond effectively. Create a fresh surface with a hand plane, jointer, or thickness planer. It’s not necessary to sand surfaces. That could actually compromise the surface’s flatness or burnish it enough to limit glue absorption.
Ideally, assemble joints promptly after making them because changes in the wood’s moisture content affect fit. A joint with a perfect test fit today could seem sloppy or overly tight when you finally get around to assembling it. Also, the wood’s surface begins to oxidize (react with the air) as soon as you cut it. In this case, remove contamination with a fresh cut or by wiping the gluing surfaces with acetone. This solvent also excels at prepping oily tropical woods such as teak.
Masking tape can provide sufficient pressure when edge-banding plywood.
Do the glue
With the exception of polyurethane and CA adhesives, apply a full wet coat of glue to both surfaces. Simply squiggling on glue isn’t enough; use an applicator to achieve full coverage. Promptly assemble joints before the glue starts to cure. Position the clamps and apply gentle pressure.
You want to see a bit of squeeze-out at the joint as evidence of complete glue application. But excessive squeeze-out is both sloppy and wasteful. Cut back on the glue applied to subsequent joints until you’ve zeroed in on the right amount.
Be cautious of cranking up too much clamping pressure, because it could squeeze most of the glue out of the joint. Called “starving a joint,” this could result in weakness and failure.
Use a series of clean rags and a constantly refreshed water supply when wiping up glue.
Expect to use different cleanup techniques for open-pore woods, such as red oak, and closed-pore woods, such as maple. With open-pore woods, immediately scoop up squeeze-out with a plastic putty knife, and then strip away the masking tape, if used. Wipe away the remainder of the glue with a clean, wet (but not sopping) rag, refreshing your water as necessary to prevent wiping a glue dilution on the wood. Finish up with a dry rag.
With closed-pore woods, wait until the glue skins over, but before it hardens. Use a plastic putty knife to remove most of the excess, and then strip away any masking tape. Use a paint scraper to lift the glue from the surface. Slightly file back the corners of the scraper to prevent gouging the wood. For squeeze-out at the intersection of parts, such as the junction of an offset rail and a leg, a gentle lifting with a chisel or chisel plane may provide the solution.
Dealing with glue goofs
If you discover dried glue beads on your workpiece, proceed with caution and a light touch. A too-heavy stroke with a paint scraper or utility knife can pry or cut out chips of wood. Instead, scrape cautiously with a cabinet scraper, or use an old chisel for removal. Dried yellow glue is hard enough to nick an edge.
For glue dried in a corner, try the chisel or detail sander. However, be careful when power-sanding hard beads of glue. The residue can quickly load the abrasive and burnish the wood’s surface.
Before staining, check for residual glue by wiping the wood with a solvent such as naphtha. Areas contaminated with glue will appear as light spots. Sand or scrape the surface, then re-inspect.
In the worst case scenario, you’ve begun to stain and exposed an area that resists coverage. The solution: Let the stain dry and then scrape or sand the problem area, inspecting your progress with a naphtha rubdown. Feather the abrasion onto the successfully stained surface to minimize transition issues.
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