Choosing the Adhesive: Open Time, Gap Fill, and What It Has to Survive

PVA (white or yellow wood glue): the default for wood-to-wood. Strong, cheap, easy, no gap fill, not waterproof unless it says so. EPOXY: two parts, sticks to almost anything, genuinely fills gaps, waterproof, and the only one on this list that will bond metal reliably. CYANOACRYLATE (super glue): instant, brittle, poor in shear and peel, excellent for tiny parts and for tacking something while a stronger adhesive cures. POLYURETHANE: expands as it cures, waterproof, fills gaps — but the foam it creates has little strength, so the expansion is not filling anything usefully. HIDE GLUE: reversible with heat and moisture, which is why instruments and antiques use it — a joint that can be taken apart in fifty years without destroying the piece. HOT MELT: fast, weak, temperature-sensitive; a jig and mock-up adhesive, not a structural one.

EPOXY fills gaps properly. It cures by chemical reaction rather than by losing water or solvent, so the bulk material in the gap is as strong as the film — which is why it is the right answer for a joint that cannot be made to fit. PVA and hide glue do not. They cure by losing water, so a thick line shrinks, cracks internally and ends up weaker than no gap at all. Adding sawdust to PVA makes a filler, not a structural joint. POLYURETHANE is the one people get wrong. It expands and the foam fills the space, but the foam is weak. It looks as if the gap is bridged and structurally it is not — and the expansion will also push a loosely clamped joint apart while it cures.

Ordinary PVA softens when wet. For anything outdoors or in a kitchen, use a water-resistant grade (D3) or waterproof (D4), or epoxy. The label matters here — 'wood glue' covers all three grades. Heat is the limit people forget. Hot melt lets go at the temperature it was applied at, PVA creeps above about 50 C, and hide glue releases with steam by design. A joint in a car, a loft or beside a stove needs epoxy or a mechanical fastening. Sustained load is the subtlest. Most adhesives CREEP — they deform slowly under constant stress — and a shelf glued with PVA will sag over years in a way it would not with epoxy or a proper joint. Where the load is permanent, let the joinery carry it and the glue merely hold it together.