How do I use polyurethane glue without it foaming everywhere?

Polyurethane glue cures by reacting with moisture in the wood, in the air and on the surface, and carbon dioxide is a product of that reaction. The foam is that gas. It has almost no strength, it will happily push a joint open if the clamps are light, and it is why people who load a joint with polyurethane end up with a foamy, weak result and conclude the adhesive is poor.

Safety

What you'll need

• polyurethane wood adhesive
• nitrile gloves
• clamps, more than you would use for PVA
• a fine mist spray or damp cloth
• masking tape to protect the surfaces beside the joint
• a sharp chisel or scraper for the cured excess

Step by step

1. Use it where it genuinely earns its place.Outdoor timber, oily and dense hardwoods that PVA struggles with, timber to masonry or metal, joints that need to be waterproof, and joints made in damp conditions. For an ordinary indoor close-fitting timber joint, PVA is stronger, cheaper and far less messy.
2. Get the joint fitting properly first.Polyurethane is often sold as gap-filling. The foam fills space but adds essentially no strength, so a poor joint filled with foam is a poor joint. Fit it as you would for PVA.
3. Dampen one face lightly.A fine mist or a wipe with a damp cloth, on one surface only. The adhesive needs moisture to cure, and dry timber in a heated room may not have enough. Wet, rather than damp, produces violent foaming and a weak joint.
4. Apply far less than feels right.A thin bead or a scraped-out film on one face is enough. The glue expands several times as it cures, so a normal PVA-sized application produces an enormous amount of foam.
5. Assemble and clamp firmly, straight away.Open time is short — often twenty to thirty minutes and less in warm damp conditions. Clamp harder than you would for PVA, because the expansion is actively trying to open the joint.
6. Mask the surfaces either side of the joint.Cured foam is hard, tan-coloured and difficult to remove from a finished surface, and it stains bare timber. Tape either side of the joint before assembly and it becomes a two-minute clean-up.
7. Leave the squeeze-out alone until it is hard.Wiping wet polyurethane smears it into the grain, where it seals the timber against stain and finish and shows up as a pale patch for ever. Let it cure fully and then pare it off with a sharp chisel or scraper — it comes away cleanly.
8. Leave it clamped for the full time.Handling strength in an hour or two, full strength considerably later, and much longer in the cold. The foam looks finished long before the adhesive is.

Tips

• The foam is the giveaway that you used too much. A well-judged polyurethane joint produces a modest ridge of foam, not a mushroom.
• It stains skin dark brown and does not wash off. Nitrile gloves, every single time, and change them when they get tacky.
• The bottle has a limited life once opened, because moisture in the air starts curing it inside the container. Squeeze the air out, cap it tightly, and expect a year at best.
• It is one of the few adhesives that will bond damp timber, which makes it the pragmatic answer for outdoor repairs in a British autumn.
• For gluing timber to masonry or to metal, polyurethane is genuinely good and the expansion into a rough surface helps rather than hinders.

Common mistakes

• Using a PVA-sized quantity — It expands several times over. You end up with a joint full of weak foam and a large clean-up job on both faces.
• Soaking the timber rather than misting it — Excess water causes violent foaming and a joint that is mostly gas. A light mist on one face is all the reaction needs.
• Light clamping — The expanding gas forces the joint open. Polyurethane needs firm, well-distributed clamping and it needs it for the whole cure.
• Wiping wet squeeze-out with a cloth — It smears into the grain and seals it. The stain or finish will not take there, and the patch is essentially permanent.

Questions people ask