How long do I need to leave the clamps on a glue-up?

Three separate clocks run during a glue-up. Open time is how long you have from spreading the glue to closing the joint. Clamp time is how long until the joint holds itself together. Cure time is how long until the glue reaches its full strength. Most glue-up failures are caused by exceeding the first and rushing the third, while people worry about the second.

Step by step

1. Read open time and treat it as a deadline for the whole assembly.It starts when the glue meets the wood, not when you finish spreading. A complex carcass with twelve joints can easily take longer than the open time of a standard PVA, and the first joints are then dead before the clamps go on.
2. Dry run anything with more than a couple of joints.Assemble, clamp and check square with no glue. Time it. If the dry run takes longer than the open time, either use a slow-setting glue or break the glue-up into stages.
3. Leave clamps on for at least the manufacturer's clamp time.Typically thirty minutes to an hour for standard PVA at normal room temperature. Longer for anything under load, anything springy, and anything at low temperature.
4. Extend the clamp time when the joint is under stress.A bent lamination, a joint being pulled together against a spring, or a slightly bowed board all need several hours. The glue creeps while it is soft, and a joint released early relaxes open.
5. Wait a full day before machining or stressing the joint.PVA reaches most of its strength in the first hours and full strength over about 24 hours. Planing, routing or sanding a green joint drags soft glue across the surface and can shift the parts.
6. Adjust for temperature — this is the biggest variable.Most wood glues have a minimum application temperature, commonly around 10 degrees. Below it the glue does not form a proper film and the joint is permanently weak. A cold garage in winter is the single most common cause of a mysterious glue failure.
7. Remove squeeze-out at the right moment.Wipe wet glue with a barely damp cloth on work that will be painted; on work that will be finished clear, let the squeeze-out go rubbery and pare it off with a chisel, because water spreads glue into the grain and shows under finish.
8. Let the piece acclimatise before final flattening.Water in the glue swells the wood locally. Level a panel a day or more after glue-up, or the joint lines sink slightly as they dry and show as ridges.

Tips

• Slow-set or extended open time PVA exists for exactly this problem and costs little more. For a complicated carcass it is the difference between a calm glue-up and a disaster.
• Polyurethane glue foams and the foam has no strength — it needs real clamping pressure and gap filling is a myth. Epoxy needs almost no pressure and genuinely fills gaps.
• Warm the workshop and the timber for a few hours before a winter glue-up, not just during it. Cold wood chills the glue on contact.

Common mistakes

• Timing open time from when you finish spreading glue — The first joint you glued has been open the whole time you were spreading the rest. On a big assembly the earliest joints can be past their open time before the clamps arrive.
• Gluing up in an unheated garage in winter — Below the glue's minimum temperature it never forms a continuous film. The joint feels solid and fails months later with no obvious cause.
• Planing the joint flush an hour after the clamps come off — The glue is not fully cured and the wood near the joint is swollen with water. Levelled now, the joint line becomes a hollow when it dries.

Questions people ask