Gluing Up a Wide Panel Without Cupping
A tabletop glued up from several narrow boards can be flatter and more stable than a single wide board of the same species, but only if the boards are arranged deliberately. Joined in whatever order they came off the stack, the panel inherits every tendency each board had and adds them together.
Start at the end grain. Each board has growth rings curving in some direction, and as it dries further it will cup away from the heart side, so the rings try to flatten. If every board in a panel is laid with its rings curving the same way, all the cupping adds up and the panel becomes one large shallow dish. Alternating the boards so the rings curve up, down, up, down turns one large dish into several small ripples, which a frame or a set of battens can hold flat and which are far less visible.
There is a competing consideration, which is appearance. Alternating the boards also alternates the face pattern, so a panel that is stable can look restless. Where the panel will be held flat by its construction anyway, it is legitimate to arrange for the best-looking match and let the construction deal with the cupping. The decision should be made knowingly rather than by default.
The joint faces themselves want to be straight rather than merely square. Two boards planed so that each edge is very slightly hollow along its length will close at the ends first and pull tight in the middle under clamp pressure, which puts the ends of the joint into compression. Ends are where a glue joint is most exposed to drying and most likely to open, so building a little compression into them is cheap insurance. The hollow wanted here is very slight, closed by hand pressure at the centre.
Clamping pressure should be even and should come from both faces alternately, since clamps on one side only will bow the panel towards them. Enough pressure to close the joint and squeeze out a fine even bead is sufficient; more than that can starve the joint by forcing out the adhesive that should have stayed in it.
Finally the panel should be left alone considerably longer than the adhesive's stated handling time before it is planed flat. A joint that is handled early may survive, but the moisture from the glue has swollen the wood either side of the line, and planing while that swelling is present leaves a shallow ridge along every joint once it dries back.