It's important to dry clamp every subassembly and assembly before applying glue to confirm joint accuracy and to be sure you've gathered all necessary clamps and protection blocks.
Three types of glue applicators: roller, paddle, and brush. A roller is best for edge joints and veneering. It wets the surface quickly and spreads the glue evenly. The great virtue of a paddle (such as a tongue depressor) is that you can shape it to suit the situation. A plumber's flux brush fits into mortise joints and the tight pin spaces in dovetail joints.
For edge-to-edge joints, clamping pressure fans out from the clamp heads at 45° in each direction. Understanding this enables you to determine how many clamps are needed.
A badly placed block, like the one on the left, causes misalignment of a mortise-and-tenon joint. Clamp each dovetail home one by one (right). The friction fit between pins and tails holds the joint together while the glue cures.

To avoid a calamity in the glue-up process, you must approach the task with the same rigorous attention to methodology that you did in selecting and preparing the material, measuring and marking out the parts and cutting the joints.

In particular, you must attend to your clamping technique because bad clamping can ruin a project. You could make an assembly with parts aligned and shoulders spot-on and then by bad clamping glue the assembly together into a permanent state of inaccuracy. However, the opposite also is true: a less-than-perfect assembly can be rendered acceptable by judicious clamping.

This lesson will address three different stages of the glue-up process: dry clamping, glue application and clamping technique.

Dry Clamping

It is essential that each subassembly and the final assembly are clamped together first without applying glue to the joints. This is called dry clamping.

Dry clamping forces you to gather all necessary clamps, clamping blocks and measuring tools. It also allows you to practice and refine your assembly procedure and accuracy checks before committing to the permanence of glue-up.

Different glue-ups require different glue applicators. Whichever one you use, the aim is the same: to thoroughly wet all of the contact surfaces on both parts being joined with a thin, uniform layer of glue. A common mistake is to think that glue applied to one surface will transfer to the other mating part during clamp-up. That's not the case. Because of viscosity and surface tension, glue will not wet easily unless it is diligently worked into the surface. Applying too much glue also is a mistake, though.

The right amount is gauged by minimal squeeze-out — no more than a thin bead at the joint line. Excess glue that drips, drizzles or oozes from the joint is a waste of money and a waste of time spent cleaning up the mess.

Different joints require different clamping techniques. Here's how to clamp some of the more common joints:

Edge-to-edge joints — Pressure from the clamp heads will fan out at about 45° from each side, as shown in the photo at top right. How much pressure to apply varies with each setup, but if you have left indentations on the board's edges, you have applied too much pressure (and possibly forced glue from the joint). Before closing the clamps, check that the joint line is tight and that the boards are aligned. Check the alignment again after tightening the clamps.

Mortise-and-tenon joints — Aligning the parts and closing the shoulder line of a mortise-and-tenon joint is more complex than with other joints. Clamp pressure must be directed at exactly the right place by using specially prepared pieces of wood called clamping blocks. Assuming that the joint has been accurately made, correct alignment of the clamping blocks with their corresponding shoulders will ensure that the assembly will check out accurately. Misalignment of the block will result in a subpar joint (see photo at right). You also must check to be sure the parts are square to one another (by measuring across the diagonals) and be sure the assembly isn't twisted.

Dovetail joints — If you make the joint so that the end grain of the pins falls short of the long grain of the tails (see Chapter 5, Lesson 4), you won't need clamping blocks. Using a brush or paddle, apply glue, assemble the joint, and clamp each dovetail home one by one (see photo at right). Don't take too long after applying glue to the long grain of pins and tails because each part quickly swells, making a tight fit even tighter. And don't try to hammer the parts home. Clamp the joint tightly together, one tail after another.