Chemical Reaction

• Polyurethane wood glue reacts chemically to moisture in the air or the product you are gluing. Due to this chemical reaction, the wood glue expands in a foaming process, fills in the voids and provides an exceptionally strong glued joint as it cures. Because it has expansive properties, polyurethane wood glue tends to work better for end-grain joints than most other glues. As the glue expands, you must keep the joints stable by clamping them.
  
  

Advantages

• Polyurethane wood glue is the glue of choice when you must join wood to wood with a waterproof joint. Waterproof and solvent-free, the glue allows for use on both nonporous and porous surfaces. The glue accepts wood stains, and it sands well even in a thin coating. Unlike polyurethane wood glue, many other adhesives work as a sealer on the wood surface, making it difficult to remove them because they dry with a texture that resists removal.
  
  

Disadvantages

• Because polyurethane glue foams as it dries, it can become messy when it spills over from the joints; unlike polyvinyl acetate (PVA) glues, it won't wipe off. Once dry, the polyurethane wood glue will not come off anything easily; sanding or cutting are the only options. Wear disposable gloves when contacting this glue to avoid touching a chemical in the glue that can cause skin discoloration. This chemical will not wash out; once it adheres to the skin, it must wear off on its own. Polyurethane glue also has a proclivity to harden in the container because moist air can become trapped when you open the can, thereby starting the hardening process and turning the glue into a solid block.
  
  

Safety and Clean Up

• Polyurethane wood glue is toxic to the touch and to ingest. Use the glue in a well-ventilated area, and wear goggles. Keep denatured alcohol, acetone or other solvents readily available to clean your hands, but avoid any skin contact with the glue. Read the manufacturer's instructions for use; each glue product is slightly different.
  
  

Types

• Different types of polyurethane wood glue are available, and some foam less than others. Additionally, some glues come in caulking gun tubes. This type of glue spreads well but has a thicker consistency, causing the dispenser nozzle to clot after the first or second use.