Use a small roller to spread a thin film of glue on both workpiece edges - never only one.

Clamping pressure fans out from the clamp heads at 45° in each direction and is transmitted to the glue line, as shown here.

Understanding pressure fans enables you to determine how many clamps are required to glue boards into a panel.

Too much pressure can bow a flat panel. Use a straightedge to check for flatness (below). If it is bowed, reduce pressure and alternate clamps top and bottom.

Inferior Edge Contact Rescued by Clamps and Glue

Total edge contact for a properly made butt joint relies on well-jointed surfaces. However, when using clamps it's easy to apply too much pressure and starve the joint of glue, assuming the joint is well made. Anything less than total contact means that final total contact occurs because clamp pressure deflects the wood: in other words, clamp pressure makes a bad joint look good. The joint usually stays tight once the glue has cured and the piece is removed from the clamps, but who knows what internal stress is required to keep the glue line closed? The fact that glue-ups usually endure despite less than perfectly jointed surfaces is a testament to the forgiving nature of the materials we use.

Glue cannot work unless it wets the wood - and that's not as simple to accomplish as it may sound. The best method is to roll a thin layer onto each surface. Roller pressure wets the wood and ensures that a thin glue layer is deposited on each face, resulting in a controlled and limited squeeze-out.

A common but definitely poor method is to squeeze a bead onto one edge and rely on clamp pressure to spread the glue. The thickness of the bead causes the pieces to slide out of alignment, and squeeze-out is often excessive and spotty. Limited or no squeeze-out indicates there may be no wetting and, therefore, no adhesion.

Best Glue Applicators to Use

Between these best and worst methods of applying glue, we typically use brushes, paddles, and even fingers as applicators. Don't use your fingers: they contaminate the glue with residual oils, making it less effective. Wet fingers also get glue in places where it shouldn't be. For a simple butt joint without splines or biscuits, a roller is the only required applicator. 

Now, onto the clamping procedure. For a typical edge-to-edge joint, we know from experience and measurements that if a clamp is placed on center across the panel, pressure from the clamp heads will fan out at about 45° from each side. In other words, there is a 90° fan that will deliver the same pressure on the glue line over the area of the fan.

Understanding pressure fans enables you to determine how many clamps are required to glue up a joint between boards of any width and length. The number of clamps needed is a function of the width and length of the two boards.

However, there's more to a successful glue-up than spacing clamps according to pressure fans. If you put clamps all on the same side of the panel, it will curve or bow under pressure. To counter this, alternate the clamps on each side of the panel.

How Much Clamping Pressure is Necessary?

How much clamping pressure to apply varies with each setup. Because pressure is controlled by two variables, muscle power and the length of the handle, I can only explain in generalities. One thing, however, should be obvious: tightened to maximum, a bar or pipe clamp exerts an enormous amount of pressure.  If you have crunched the clamp heads into softwood boards 3/4" thick or deflected the fibers on the edge of 3/4" oak boards, then you have applied too much pressure. Apart from the obvious damage to the boards, so much glue has probably been forced from the joint that it's at risk of "glue starvation" and possible joint line failure.

Before closing the clamps on any edge-to-edge assembly, you must check that the joint line is tight and that the boards are aligned. Put one board in the vise or stand it on edge on the bench and put the second one on top. Using a straightedge, check that the two boards are aligned. Once you have glued and clamped the joint, confirm that the alignment is retained. Making a distorted panel from two flat boards is one of the commonest errors in woodworking.