Seal Miter Joints Before Glue-Up
I’ve heard that sizing a miter joint before glue-up strengthens the joint. Is that true, and how should I apply it?
Jo Jetsy, Mount Sinai, NY
Glue sizing does strengthen the joint, because it slows down the capillary action of the end grain. Standard woodworking glues (yellow and white PVA glues) are a combination of water and solids. As the water evaporates, the solids bind to one another. This becomes a problem when the glue is applied to end grain, like on a miter joint. Looked at from the end, a piece of wood is like a bundle of straws. When you put glue on the joint, they immediately begin to pull moisture (and some solids) away from the surface and into the wood. That causes the glue to dry too quickly and leaves behind a smaller amount of solids, resulting in a weaker joint.
Sizing the joint slows down the capillary action and makes a stronger joint possible. I recommend making the size from 50% glue and 50% water. Brush it on the joint, let it dry overnight, and the next day, assemble it as you normally would.
Thin some glue. A mixture of equal parts water and glue is thin enough to be absorbed readily by end grain.
Size the joint. For the strongest joint, let the mixture soak into the fibers and dry before gluing and assembly.