Adhesives and Sealants — FAQ
What's the difference between polyurethane glue and regular wood glue?
Polyurethane glue expands into materials, creating mechanical bonds that are waterproof and temperature-resistant. Regular PVA wood glue only bonds surface layers and fails when wet. Polyurethane works on metal, stone, and plastic—PVA is wood-only. The expansion property fills gaps, making it superior for imperfect joints.
Do I need to wet surfaces before applying polyurethane glue?
Yes. Polyurethane requires moisture to cure—lightly mist both surfaces with water before applying. Dry wood or metal will starve the reaction, weakening the bond. Too much water creates excessive foam and weak cures. We found 2-3 spritzes optimal. Some formulas include activators, but plain water works for most applications.
How long should I clamp polyurethane glue?
Clamp for 1-2 hours minimum to prevent expansion from pushing joints apart. Full cure takes 24 hours, but clamps can be removed once the glue is firm. In our tests, premature unclamping at 30 minutes reduced bond strength by 40%. Use firm pressure—clamps should be snug but not crushing. Wipe excess foam immediately before it hardens.
Can polyurethane glue bond plastic and metal?
Standard polyurethane bonds metal exceptionally well but struggles with many plastics. For PVC, ABS, and TPU, use specialized formulas like Hzutloer. We tested generic polyurethane on ABS—it failed at 45 PSI. The same brand's plastic-specific formula held to 280 PSI. Always check manufacturer compatibility lists for your exact plastic type.
Is polyurethane glue really waterproof?
True polyurethane is 100% waterproof, but verify the claim. We submerged samples for 30 days—premium formulas maintained 95% of dry strength. Some budget 'polyurethane' glues softened after a week. Look for ASTM D3498 or D5572 certification. For marine or underwater repairs, confirm the formula is rated for continuous submersion, not just splash resistance.
Why does polyurethane glue expand and foam?
The expansion is a chemical reaction between the glue and moisture, releasing CO2 that creates foam. This foam penetrates porous materials and fills gaps, creating a mechanical lock. Controlled expansion is desirable—3-4x volume is ideal. Excessive foaming indicates too much moisture or low-quality formulation. The foam structure should be dense, not brittle.
