Every workshop has a bottle of yellow glue sitting somewhere near the bench. That's the right answer for most furniture work, which is probably why nobody thinks very hard about it.
The problem shows up when the joint doesn't fit PVA's profile — oily species, outdoor piece, end-grain bond, a joint that needs to come apart someday. The failure mode isn't usually choosing a bad adhesive. It's using a good adhesive in the wrong place because it was there.
Four families. Here's when each one is right.
PVA — the default
Yellow PVA (Titebond Original, aliphatic resins generally) cures by water evaporation. The polymer chains lock in place as the water moves into the wood and leaves. On long-grain-to-long-grain furniture joints with proper surface prep, PVA creates a bond stronger than the surrounding wood fiber. The glue line is not the weak point. The wood is.
Open time at 70°F: 5–8 minutes. In a hot dry shop in August, that clock moves faster. In a cold shop in November it moves slower, but cure time lengthens, and below 50°F the bond degrades. Know your shop temperature.
Titebond II adds water resistance. Titebond III adds waterproofing and a longer open time. For interior furniture that stays dry, you don't need either upgrade.
What PVA doesn't do: fill gaps, bond end grain reliably, or resist sustained cyclic load without creeping. Chair seat-to-leg joints take racking stress every day. PVA is a thermoplastic — its cured film deforms slowly under heat and sustained load. The joint needs to fit so well that geometry carries the load, not the glue.
Hide glue — the reversible one
Hide glue is collagen dissolved in water, applied hot. The key spec is gram strength — it measures gel stiffness. Higher gram strength, faster set and harder cure. 192 gram is the standard furniture grade. 251 and above for applications where fast grab matters.
Hot hide glue runs at 140–150°F. Application temperature isn't optional — too cool and it gels before the joint closes; too hot and the protein degrades. Open time at 192 gram in a 70°F shop: 60–90 seconds. In a cold shop, the joint itself chills the glue the moment it makes contact, and you have less time than that.
The reversal is the point. Heat and moisture release the joint cleanly — a hot damp cloth does it. This is why you use hide glue on antique furniture repair (the piece was built with it; the joint structure expects it), on instruments (tops need to come off for repairs), and on anything you intend to service over its lifetime. The moisture sensitivity that enables reversal also enables failure in uncontrolled environments. Outdoor furniture and unheated spaces are not hide glue applications.
Liquid hide glue (Old Brown Glue and equivalents) trades some strength and a crisper cured film for room-temperature convenience and 15–20 minutes of open time. The reversal advantage holds.
Polyurethane — the gap-filler
PU cures by reacting with moisture in the wood. The cure expands — typically 30–50% volume increase. The foam is not a defect. It's the mechanism.
That foam fills gaps. Slightly loose tenon, panel edge that doesn't quite close — PU handles it and still produces a structural, waterproof bond. For outdoor furniture and high-oil tropical species (teak, ipe, cocoboo), PU is a legitimate choice. Wipe mating surfaces with IPA immediately before application to clear surface oil.
What PU doesn't do: sand cleanly, leave a clean glue line, or stay under control on a tight joint. For interior furniture with well-fitted joints, it adds mess without benefit. Stay with PVA.
Epoxy — the specialist
Epoxy is two-part thermoset: resin and hardener, fixed ratio, chemical cross-linking. No solvent evaporation, no moisture dependence. No creep, full stop.
Use slow-cure formulations (30-minute or overnight) for structural joints. Five-minute epoxy trades strength for speed — the cross-link network is incomplete. It's a repair adhesive, not a joinery adhesive.
Epoxy is correct in three situations that nothing else handles as well:
Oily species. Teak, ipe, cocoboo, rosewood. PVA adhesion is unreliable on high-extractive-content species. Epoxy isn't. Wipe mating surfaces with acetone immediately before application. Apply within a few minutes of wiping.
End-grain structural bonds. PVA doesn't penetrate end grain effectively. Epoxy does. First coat: thin slightly with the manufacturer's recommended thinner to improve penetration, let it tack, then apply full-strength for bonding.
Gap filling and void filling. Solid fill, machinable, finishable. For large pours in knots or live-edge voids, use slow-cure to manage the exothermic heat — thick epoxy generates enough heat to crack surrounding wood if the mass is too large.
One caution: some film finishes have adhesion problems over cured epoxy. Scuff-sand epoxy fills and apply a wash coat of dewaxed shellac before topcoating if adhesion is uncertain.
The selection rule
Four questions, in order:
- What's the load? Long-grain furniture joints → PVA. End-grain structural bonds → epoxy. Cyclic racking on a chair → PVA with a joint fit that takes the load geometrically, not adhesively.
- What's the species? Cabinet hardwoods → PVA. Oily tropicals → epoxy or PU. Anything the piece will be repaired or serviced → hide glue, if environmental conditions allow.
- What's the life context? Outdoor or high humidity → PU or Titebond III. Antique repair or instrument → hide glue. Interior furniture with controlled environment → PVA.
- What's the fit? Tight, properly prepared surfaces → PVA or hide glue. Legitimately loose joint that needs to close → PU or epoxy depending on the other answers.
Using slow-cure epoxy on a tight-fitting walnut mortise-and-tenon going into a dry interior cabinet is not superior craftsmanship. It's over-specified. The joint doesn't need it, the epoxy adds cleanup complexity, and the joint can't be serviced. PVA is correct.
Use the simplest adhesive that handles the load, the species, and the context. That's it.