Which wood glue do I need — PVA, D4 or polyurethane?

Wood glues all look alike in the tube and the numbers on the bottle are the only real specification. Understanding what they mean removes almost all the guesswork.

Step by step

1. Understand the D classes.D1 is dry interior only. D2 is interior with occasional short-term damp. D3 tolerates frequent short-term water and protected exterior use. D4 handles frequent exposure to running or condensed water.
2. Use ordinary PVA for interior work.D2 or D3 white or yellow PVA covers furniture, joinery, panels and repairs. It is strong, cheap, cleans up with water, and a well-made joint is stronger than the timber.
3. Step up to D4 outdoors.Garden furniture, gates, exterior joinery. D4 PVA is water resistant rather than waterproof, so the joint still needs to shed water rather than sit in it.
4. Consider polyurethane for damp and mixed materials.It cures by reacting with moisture, foams slightly to fill small gaps, and bonds to timber, some plastics, stone and metal. It is fully waterproof once cured.
5. Accept polyurethane's drawbacks.It foams out of the joint and the foam has no strength, it stains skin brown for days, and it needs proper clamping because the expansion can push joints apart.
6. Use epoxy for gaps and non-porous surfaces.Two-part epoxy fills gaps with full strength, bonds to metal, glass and cured finishes, and is waterproof. It is expensive and needs accurate mixing.
7. Use hide glue for antiques and instruments.Reversible with heat and moisture, which is exactly why conservators use it. Any future repair can undo the joint without destroying the piece.
8. Check the open time against the job.Standard PVA gives five to ten minutes. A complicated glue-up needs an extended open time version, or you will be clamping partly set glue.
9. Check the minimum temperature.Most PVA needs five or ten degrees minimum. Below that it chalks and never reaches strength, which is the classic winter garage failure.
10. Match the colour to the work.White PVA dries near-clear, yellow aliphatic dries pale amber and sands better, polyurethane dries tan and foams. On a clear-finished piece the colour of a glue line matters.

Tips

• The D number is water resistance, not strength. A D2 joint indoors is as strong as a D4 one.
• Polyurethane foam squeeze-out has no strength at all — do not mistake it for a filled gap.
• Cold is the commonest cause of a failed PVA joint. Check the minimum temperature on the bottle in winter.

Common mistakes

• Assuming a higher class number means a stronger glue — The classes describe resistance to water only. In dry conditions all of them exceed the strength of the timber, so paying for D4 indoors buys nothing.
• Relying on polyurethane's foam to fill a gap — The foam is full of voids and has almost no structural strength. It looks like a filled joint and behaves like an empty one.

If it doesn't work

Questions people ask