3 Must-have Adhesives
You'll seldom need to reach for another adhesive if you stock your shop with a weatherproof polyvinyl acetate (PVA) glue, slow-set epoxy, and medium-viscosity cyanoacrylate (CA). Here's all you need to know to select and use them.
Choose your adhesive
Let's take a look at the types of jobs at which each choice excels.
Type II PVA
This is the go-to glue in a woodworking shop, and the least expensive of the three at about $5 for 8 ounces. Pick it for joinery, gluing up panels, flat laminations, and applying veneer. An open time of about 10 minutes provides sufficient time on most glue-ups for arranging parts and getting clamps in place. A Type II-rated glue resists moisture, so it holds up on indoor and most outdoor projects. It also maintains some flexibility after it dries to accommodate wood movement.
This two-part adhesive requires mixing a resin and a hardener before use, above. It bonds metal and some plastics to wood, and its gap-filling ability can remedy less-than-perfect fits. This adhesive is truly waterproof, exceptionally strong and, with an open time approaching 60 minutes, you won't have to rush through complex assemblies. It cures hard, resisting springback, making it a good choice for bent laminations, below. A quart of resin and a pint of hardener cost about $60 for both ($10 for 8 ounces).
Also known as super or instant glue, CA sets in about a minute, making it a good choice for small, difficult-to-clamp assemblies, and for reattaching chip-out, so you can keep working. If you need a grip in a matter of seconds, spray the joint with accelerator, typically sold alongside the glue.
A medium-thickness formula, about the consistency of thin syrup, won't squeeze out of tight-fitting joints. If a workpiece soaks up the glue too fast, leaving little on the surface, simply apply another coat.
CA cures hard and brittle compared with PVA, but that brittleness can be an advantage if you have pieces to join temporarily. For example, you can attach a scrap block to a bowl blank before turning it. A sharp rap with a mallet will shear the glue block from the bowl after turning. CA is the most expensive choice of the three at $10 for 2 ounces ($40 for 8 ounces).
Prep your material
To provide the best bond, PVA and CA glues need very smooth surfaces that fit together with no gaps, below. Moisture content of the wood must be below 15% (most kiln-dried hardwoods fall below 8%), and the temperature of the materials and glue must be above 55°F.
Although epoxy fills gaps, it does not accept finish, so visible joints should still fit tightly. Epoxy requires a bit of "tooth" in the faces being bonded, so scuff smooth surfaces, below. Wipe metals and oily woods, such as teak, with isopropyl alcohol or acetone to remove oils and contaminants. Allow the surface to dry before applying the epoxy. Check labels for details, but typically, the temperature of the materials and adhesive should be above 50°F.
Before applying glue, dry-assemble the workpieces to check the fit of joinery, and to determine how you'll clamp the assembly, below. Doing this allows you to identify and correct any ill-fitting joints or parts, and areas of potential glue squeeze-out [Wipe out squeeze-out, below].
Apply the adhesive
It may seem obvious, but any surface without adhesive won't stick to another surface. So don't rely on clamping pressure to spread a bead of glue around a joint. Instead, fully cover one of the mating surfaces with an even coating, below. Apply adhesive to face and edge grain; porous end grain draws glue in, starving the surface. If you must glue end grain, first appy a thin coat of adhesive to seal the surface. Allow it to penetrate a few minutes, then apply a second coat of glue.
Use a flux brush to cover narrow areas and reach into crevices, below. A silicone brush [Sources] or old credit card works well for edges and moderately sized surfaces. For large areas, roll out the glue with a rubber roller [Sources].
Put it under pressure
As you tighten clamps on an assembly, apply just enough pressure to bring the workpieces together or close the joint, then add another quarter- to half-turn of the handle. Excess pressure squeezes the adhesive out of the joint, weakening it. Check parts for proper alignment and make any adjustments before the glue begins to set. Leave PVA- and CA-glued assemblies clamped for at least one hour, and epoxy for at least six hours. On assemblies under stress, such as a bent lamination, leave the clamps on for 12 hours. Allow 24 hours for a full cure on all three adhesives.
Wipe out squeeze-out
A thin bead of squeeze-out indicates a properly glued joint. But stray adhesive will show under a finish, and you don't want hardened beads or drips as a permanent part of your project. Scraping or prying away dried glue can take chunks of wood with it. Follow these strategies to reduce or eliminate potential problems.
To soften dried PVA, dip a paper towel into a mixture of equal parts acetone, water, and vinegar. Wring out the towel, place it over the squeeze-out, and cover it with plastic wrap or a plastic bag. After soaking for a couple of minutes, the softened glue should scrape off easily.
With PVA glue, allow squeeze-out to happen, then peel it off with a putty knife or chisel after the glue turns rubbery (about 30–40 minutes).
Remove excess epoxy immediately after clamping by wiping with a rag dampened with acetone or lacquer thinner. Roll the rag as you work to avoid smearing the adhesive.
Cutting pieces oversize allows cleanup as you trim the piece to final size. This lamination, shown in the glue-up above, has been jointed on the edges. Crosscutting to length removes the squeeze-out on the ends.
CA glue dries so quickly, squeeze-out doesn't have much time to penetrate the workpiece. Simply allow the adhesive to cure, then scrape and sand away the excess.
Eliminate squeeze-out on moldings and edging by routing a shallow V-groove near each edge to capture excess glue.
Prevent squeeze-out from reaching the wood by applying painter's tape around joints, and areas that will be hard to reach after assembly, such as inside corners. Dried adhesive peels away with the tape.