Squeeze-out can and should be avoided
Bill Tindall, E.Tn.
One of the more useful things we learned while working in the Headley shop was to avoid squeeze-out. It is avoidable. I never get glue where it should not be on mortise and tenon or dowel joints, or the inside corners of dovetail joints. On panels there will be only tiny droplets of squeeze-out that are easily scraped and sanded away.
Glue theory: For a perfect joint only 3 molecular layers of glue are required to make as good a bond as can be made. Any more is gap filling which is weaker. Once wet with glue no clamping force or scraping can remove the surface layer of glue from the wood. Hence, once wet with glue there will always be enough glue on the surface to make as good a bond as can be made.
Gluing begins with proper joint preparation. For a mortise and tenon or dowel joint the female part of the joint has extra depth to accommodate glue scrapped from the walls as the male part is inserted.
Most critical is glue application. For edge gluing I apply glue straight from the bottle. For 4/4 lumber I lay down a double stripe as shown in the picture. For thicker lumber I will add a stripe down the center of the mating piece. The fist step of assembly is to rub the joint to transfer and distribute glue on each mating surface. Clamp pressure is applied slowly so that glue has a chance to flow into any place along the joint where glue isn't.
For mortise and tenon and dovetail joints glue is applied to both surfaces with a short bristle brush or spatula. For a dovetail a thin layer of glue is applied to the tail board and glue is brushed or scraped onto the pin board to leave the surface just wet with glue. Upon insertion of pins into tails excess glue is scraped from the tail sockets and squeezed to the outside of the joint where it can be removed after cure. The just wet pins do not have any excess glue to be scraped so no squeeze-out appears on the inside corner.
For mortise and tenon excess glue is applied to the mortise with a spatula. These joints are often gappy, more or less, so extra glue to fill the gaps is desirable. Glue is then applied to the tenon with brush or spatula to just wet it. As is the case with a dovetail there is not excess glue that will scrape to the outside of the joint upon insertion. Note that in each case the last glue to be applied is the very thin layer on the male part of the joint. It is essential that this layer not begin to dry before insertion. In either case it is impossible to scrape glue off the mating surfaces upon insertion to the point where the bond would suffer.
