Polyurethane wood glue

Polyurethane wood glue is usually a one-part liquid prepolymer that reacts with moisture. You dampen one surface when the instructions require it, apply glue to the other, assemble and clamp. The reaction makes a crosslinked polyurethane and often releases gas, so the glue foams and expands from the joint. That foam is not a strong substitute for fitted timber. It can fill visible space, but the most reliable bond still comes from close contact and firm clamping. The chemistry will cure between non-porous materials if enough moisture is present, which gives it a wider substrate range than PVA. It also gives good exterior water resistance in approved grades. It suits timber, stone, ceramic, rigid foam and selected metals or plastics when the product lists them. The long open time helps complicated assemblies, but squeeze-out is messy and stains skin and porous timber. Let excess foam cure, then cut or sand it away. Avoid flooding the surface. Polyurethane glue is a poor choice where foaming would move the parts, where the glue line must stay invisible, or where a flexible joint is needed. It can bond the face of oily timber poorly. Waterproof does not mean suitable for continuous immersion. Food contact, drinking water and structural building use still require a named approval.

Exterior and interior timber joints; mixed-material repairs involving wood, stone, ceramic, rigid foam or selected metal; laminations and awkward assemblies that need a longer open time; jobs where a moisture-curing adhesive is useful between less porous faces.

Dry-fit and correct the joint first. Remove oil, loose fibres and dust. Dampen one face only as directed; too much water can make excessive foam. Apply a thin film to the other face, assemble within the open time and clamp evenly for the stated period. Protect surrounding porous surfaces from squeeze-out. Cut away cured foam and sand before finishing.