Wood Glue Problems: Failed Joints, Starved Glue Lines and Repairs

Symptoms

• The joint came apart cleanly along the glue line
• A thick, visible glue line or a gap
• Glue still soft hours later
• A joint slowly opening under a steady load
• Glue lines sunken below the surface after planing and finishing
• Pale patches where stain did not take beside a joint
• Foamed polyurethane squeeze-out on a show surface
• An old furniture joint gone loose

Likely causes

Fixes

Glue makers report bond tests with two numbers: the strength, and the 'wood failure' — the share of the broken area that is torn wood rather than adhesive. Titebond's own figures for Original on hard maple are 3,600 psi with 77% wood failure. A joint that broke with wood torn from both faces held at the glue line. A clean separation, with glue on one face and bare wood on the other, means the glue never bonded properly to that face: a surface that was stale, glazed or oily, glue that had skinned before the joint closed, wood that was too wet, or a shop below the glue's lowest temperature. A thick line of glue in a gap means the joint did not fit — the Wood Handbook says most wood adhesives do not fill gaps well. A starved joint, with little glue left on either face, comes from the opposite fault: the Handbook ties it to pressure so high, or adhesive so thin, that it was squeezed out or soaked too deep.

• Still soft. Titebond says moisture above 10% slows its water-based glues and above 16% may stop them drying; each glue has a lowest temperature for glue, air and materials. Polyurethane is the exception: it needs some moisture to cure.
• Sunken glue lines. Water from the glue swells the wood along the joint; plane it too soon and the line shrinks below the surface later, which the Handbook says shows plainly under a gloss finish. It suggests conditioning for 7 days at room temperature before surfacing; Titebond says water-based glues can take 24 hours to cure before machining.
• Pale stain patches. Dried glue left on the surface keeps stain out; Titebond's instruction is to scrape off and sand dried excess before finishing.
• Foam. Gorilla's polyurethane expands as it cures; its maker says to remove cured squeeze-out by sanding, chiselling or scraping.
• Creep. The Handbook says PVA joints yield under continued stress; a joint loaded all the time may slowly open.

A loose joint is taken apart the way its glue allows. Titebond says heating an Original, II or III joint with a heat gun or hair dryer reduces the glue's strength, and that steam from an iron may also work; a few drops of water absorbed at the edge of a joint made with its liquid hide glue weaken it, and a few drops of acetone may loosen its Instant Bond cyanoacrylate. Conservators favour glues that can be undone later: the furniture conservator Norbert Gutowski describes hide glues as strong and reversible, re-treatable with moisture, and PVA as normally curing irreversibly, though steam can soften it. The makers differ on old glue. Old Brown Glue says its hide glue bonds to itself; Titebond says its glues are designed for bare wood, so a joint glued before with a PVA is cleaned back to fresh wood before regluing. Either way the joint must fit again, or the gap needs a gap-filling adhesive.

When a joint fails again after a good glue-up, the joint may be the problem, not the glue — an end-grain joint that cannot hold, or one fighting wood movement. See choosing a wood joint, mortise and tenon joints and clamps. The adhesive families are compared in the glue guide.