applying glue to sliding joints
Bill Tindall, E.Tn.
Glue application to dovetails, and related other slid together joints, came up in a conversation below. I would like to continue it separately.
I think of three goals for applying the glue to a dovetail or mortise and tenon joint- (1)if liquid hide glue, to lubricate the joint, (2) achieve wetting of both surfaces, (3)prevent glue from squeezing out where it would cause a problem in finishing. In the discussion below it became clear that subtle details affect one’s preference for how the glue is applied.
I use the equipment shown. I can pick up a drop of glue with the flat end of the spatula and rapidly wipe a thin film on the long grain of a dovetail or pin. Because I am only applying a thin film there is little excess glue to squeeze out. Any that squeezes out is forced to the outside of the joint where it will be removed during finishing or in the case of a drawer, fitting. If I was not able to rapidly apply a thin film my application details would be different.
A joint will only bond where the glue has wet both mating surfaces. If glue is applied to only one surface, one must rely on excess glue from the applied surface wiping off on the “dry” surface. If the mechanics of sliding a joint together are considered, excess glue is forced to the outside. As the joint is slid ever more together there is less glue available to transfer. It would seem likely that with a tight joint there would be no transfer after even a partial engagement of the parts.
End grain to long grain joints are weak. Personally, I see no point in applying glue to the end grain of a dovetail joint. I would especially avoid the bottom where any excess will squeeze out into the interior of the joint where it is hard to remove. Because I often stain wood I am paranoid about avoiding squeeze out to areas that will get stained.
