Q: How come I have to put yellow glue on both boards of a glue-up? I like to just slop it on to one edge, clamp the two together and wipe up the squeeze-out.
A: You might be OK using the method you describe, but I’d take a little more time and put a thin line of glue on each surface. A successful PVA glue-up proceeds as follows: glue is spread on the surface of both pieces of wood where it penetrates and glue molecules begin to react with cellulose molecules. If any part of the surface of one board looks dry, add some more glue. The boards are then clamped together and excess glue is squeezed out. The thin line of glue on the mating surface of one board bonds with glue on the surface of the other board. Some glue has bonded with cellulose molecules in the wood, and some glue is bonding with other glue molecules. Both adhesion (glue bonded to wood) and cohesion (glue bonded to glue) must be present for a strong joint.