Fixing Glue Failures
by Tobe Walter
Fixing Glue Failures
Accurate woodworking often calls for cutting, planing and chiseling to a line and not beyond. Gluing up is really no different: You must apply enough glue to avoid starved joints, yet not so much that you will make a mess.
Even if your glue bottle dispensing technique is flawless, there are many factors that can hinder successful glue ups. Bad joints can result from glue contamination, improper curing or resins in the wood itself. Such difficulties can also cause blisters and delamination in wood veneers. Glued-up sides and tops can fall apart if the edges being joined have been glazed by dull tools during machining. Worse, a whole assembly can fall apart if the glue is bad or if your shop temperature plummets before the glue has cured. Like other woodworking dilemmas, gluing disasters can be remedied if you take the appropriate steps.
Glue starved joints Joints of obvious poor fit are usually repaired soon after they are cut. But, sometimes, bad joints aren't discovered until after the parts have been assembled. In the haste of gluing up, or in an effort to avoid excessive glue squeeze out, you might apply too little glue and end up with starved (dry) joints. When the surfaces of two mating parts haven't had enough adhesive spread on them, they won't bond properly. Starved joints can also result
from not allowing enough time prior to clamping for the adhesive to soak into the wood, or from too much clamping pressure. In either case, the glue squeezes out before the bond is achieved. And often, starved joints occur during the hurried assembly of a complex piece a big face frame or chair legs and stretchers. When many parts need to be coated with glue, assembled and clamped before the glue sets, glue is applied hastily, and a joint or two might not get a proper application.
Starved joints are considerably weaker than properly bonded ones. You can often spots starved joint by the lack of glue squeeze out. Ideally, every joint should have a tiny row of glue beads at the juncture of the mating parts. A starved joint is likely to make itself known the first time you test a newly assembled chair: You'll feel the legs and stretchers rack. And a starved edge-to-edge glue joint in a panel often can be separated just by applying twisting force on either side of the joint line with your hands.
The strongest way to repair starved joints glued with hide or PVA glue is to disassemble the piece, reapply glue (this time more liberally) and clamp it up again. This approach works with glued joints that have not had a chance to cure, which takes about 24 hours.