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Repairing Wood: How to Make Strong Glue Joints in Wood

How to glue wood joints that are stronger than the wood itself.

FH04MAY_GLUJOI_01-2Family Handyman
Repair broken furniture and cracked woodwork, make stronger woodworking projects, and learn how to make cleaner, tougher glue joints with these gluing tips and techniques.

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Apply tape to control glue squeeze-out

Cleaner, stronger joints

Glue squeeze-out soaks into the fibers of raw wood, leaving blemishes when you later apply the finish. The usual solution for this is to clean it off with a wet rag or sponge. But too much water around the joint can weaken the bond. It’s better to stick down masking tape along both edges of the joint before gluing. The excess glue will then squeeze out onto the tape instead of the wood, and you can just peel the glue away when it’s dry.

How to Fix Cracked Wood: Use tape to clamp cracked wood

Photo 1: Tape one side

Check the fit first, then stick the ends of transparent sealing tape to the underside of the larger piece being repaired.

Photo 2: Fit pieces together

Apply glue to the smaller piece and fit it into place. Then wrap the free ends of the tape tightly around the piece. Transparent tape allows you to see the joint so you can get it perfectly aligned.

It’s often tough to repair cracked cabinets and furniture with regular clamps. But transparent tape makes a great substitute. If the wood is just cracked, flatten the end of a drinking straw and blow the glue into the crack. Then tape it.

If a piece has broken off, follow Photos 1 and 2. One drawback: Super-sticky tape can pull off finishes and paint when you remove it. Use light-duty tape or adhere regular tape lightly. And remove it as soon as the glue dries.

Clean and clamp when re-gluing a chair

Photo 1: Clean dowels

Clean all the old glue off the dowel ends by scraping with a utility knife. Don’t remove any of the wood or your re-glued joint will have gaps and be less strong.

Photo 2: Ream out sockets

Remove old hardened glue from sockets with a round wire brush. Scrape old glue from the socket bottom with a narrow chisel.

Photo 3: Re-glue and clamp

Clamp the joints to ensure a tight bond. Improvise using a combination of elastic cords, clamps, wood scraps and other devices to clamp the entire seat/leg assembly first.

Regluing a chair is challenging because you usually have to at least partially disassemble the chair and glue the same joints all over again. One critical step is to clean off every bit of the old glue. There will be quite a bit of it, since you’ll probably have several loose joints and may have to knock others apart to disassemble the loose ones. You have to clean both the dowel end (Photo 1) and the socket (Photo 2). The trick is to do it without digging into the wood. The more wood you shave away, the larger the gaps that the new glue will have to fill. Use sandpaper only as a last resort, because it tends to sand away wood as well as glue.

You’ll find the steel brushes for cleaning the sockets (Photo 2) in the plumbing section of a home center or hardware store. They’re designed to clean the insides of copper pipe and fittings and are available in several sizes.

Clamping may call for creativity (Photo 3). The trick is to first dry-fit all the parts you intend to glue at one time. If possible, glue the chair in two stages: the seat and legs first, and then when they’re dry, the backrest section. Test-fit the clamps to make sure where every clamp will go. Then you can work swiftly when applying the glue. Even so, use liquid hide glue rather than yellow glue; hide glue gives a much longer open time before “grabbing,” so you can get all the parts and clamps in place.

Add glue blocks

Reinforce corners

When the backside of a joint is out of sight, glue blocks make great reinforcement. Cut 1/4-in. x 1/4-in. strips of wood, then cut the strips into shorter lengths. Use plenty of glue on each contact surface, and press the blocks firmly in place where they won’t interfere with a drawer’s movement. This is one of the few times you won’t need clamps when gluing, since there’s very little stress on the joint.

Glue and screw whenever possible

Photo 1: Predrill

Clamp the part in place, then drill a clearance/countersink hole in the top piece, and a pilot hole in the bottom piece. With a combination drill/countersink bit, this drilling can be done in one operation.

Photo 2: Glue

Apply the glue to the bottom piece. Let the glue sit for a minute before you join the parts, since the glue soaks more deeply into end-grain fibers.

Photo 3: Drive in screws

Place the screws in the holes of the top piece so their points project slightly through the bottom. Align the points with the pilot holes for proper positioning, then drive the screws tight. Wipe off glue that squeezes out.

Screws are ideal for joints that call for extra strength, or where accurate positioning of a glued piece would be difficult.

Insert glue biscuits to double joint strength

Photo 1: Cut slots

Cut oval slots with a biscuit joiner. Use a guide to carefully align the slots to match the slots in the pieces to be joined.

Photo 2: Glue

Apply glue to all surfaces of the slots on both pieces. Insert the special biscuits in one side.

Photo 3: Clamp

Slide the joining pieces over the biscuits, square up your project and clamp all joints firmly.

They may not be good to eat, but glue biscuits are the do-it-yourself cabinetmaker’s best friend. A biscuit joiner (Photo 1) is very user friendly and simple to operate. It cuts precise oval slots in the ends and the surfaces of wood parts that enable you to position the parts for clamping and gluing quickly and accurately. The glued-in-place biscuits provide broad gluing surfaces that make for a strong joint. A special glue bottle (Photo 2), available from woodworking suppliers, speeds up the application of glue and distributes it evenly in the slots. Don’t delay during glue-up! Biscuits swell after gluing (which adds to their strength), so you don’t have much time for assembly. Preassemble with dry biscuits to check fits—you won’t get a second chance.

Spread glue evenly on large areas

Use a glue spreader

While a notched glue spreader is the most effective and neatest tool for spreading glue on a flat surface, you can use an old credit card in a pinch. Draw it lightly over the glue to leave a thin film. For spreading glue on smaller or curved surfaces, you can buy stiff-bristled 1/4-in. throw-away brushes in the home center plumbing department. Or use the time-honored finger; just make sure you wipe your finger clean before you mess up that masterpiece you’re building.

Reminders for strong glue joints

  • Make sure your gluing surfaces are clean and smooth but not too glossy. Rough surfaces don’t allow enough glue contact. Glossy surfaces prevent the glue from penetrating the fibers and getting a good grip. You can sand lightly to smooth roughness or remove gloss, but don’t try to actually shape a joint by sanding; it’s impossible to get a good mating fit that way. If you’re sanding, use a block and be careful not to round over sharp edges.
  • Clamp all glued joints. Pressure is necessary to form a tight, gap-free bond, and to help force glue into the wood fibers. Clamping also prevents movement while the glue is hardening. In situations where you can’t use clamps, use screws, elastic cords or weights.
  • Do a dry run with clamps before you apply any glue. This not only allows you to check for a good fit but also ensures that you will have your clamps adjusted to proper length, and all other necessary tools at the ready. It’s important to complete a glue-up fairly quickly; even though it takes about an hour for most wood glues to set and 24 hours to cure, the initial “grab” takes place in two or three minutes, and clamping should be completed by then.
  • Get a good fit between the two glued surfaces. Wood glues (except for epoxy) won’t bridge gaps, so any joint with gaps will be weak. The parts should fit together snugly. If you can’t reshape the part with a router or table saw, try gluing thin wood curls in place to fill the gaps. (You can cut curls from a scrap board using a wood plane.)

Choose the best type of glue

Yellow exterior glue (sometimes gray)

Use it for outdoor projects, but not continuous submersion. Will be labeled “water resistant” or “exterior.”

White and yellow interior glue

The most common “workhorse” wood glue. Not for outdoor use.

Liquid hide glue

Use it for furniture repair; very long open time for assembly—up to 30 minutes. Requires a long curing time.

Polyurethane glue

Use it when you need a completely waterproof glue. Also glues metal and some plastics. Long open time for assembly—up to 15 minutes; curing time: up to five hours.

Epoxy

Use it when you need to fill gaps and for great strength. Comes in two parts that must be mixed just before using. Epoxy will glue most materials, and it’s waterproof. Won’t wash off your skin—or anything else.

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

  • Biscuit joiner
  • Clamps
  • Cordless drill
  • Countersink drill bit
  • Rags
  • Rubber mallet
  • Utility knife
You’ll also need a glue spreader, a biscuit-slot glue tip, a round wire brush, and elastic cords.

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.

  • Clear sealing tape
  • Glue
  • Masking tape
  • Sandpaper