How do I flatten a glued-up panel without a planer?

A panel wider than your machines is the classic small-workshop problem, and the reason people struggle is that they start planing before they know what shape the panel is. There are only three faults — twist, cup and bow — and each is removed differently. Removing them in the wrong order means doing the work twice.

What you'll need

• A jack or fore plane, or a router and a flattening bit
• Winding sticks, or two identical straight battens
• A long straightedge
• A flat surface to work on
• A smoothing plane or a random orbit sander

Step by step

1. Put the panel on a flat surface and see how it sits.If it rocks on two diagonally opposite corners it is twisted. If it rocks along its length it is bowed. If the edges lift and the middle sits down, it is cupped.
2. Read the twist with winding sticks.Two identical straight battens laid across each end of the panel. Sight over the near one at the far one — if they are not parallel, the panel is in wind, and the direction tells you which corners are high.
3. Mark every high area with pencil scribbles.You are working to remove pencil, which is far more reliable than judging by eye. Re-check and re-mark as you go.
4. Take the twist out first.Plane the two high diagonal corners down, working diagonally across the grain with a cambered plane iron so the plane rides high spots and skips hollows. Check with the sticks every few passes.
5. Then take out the cup or bow.Work across the grain to remove material quickly, then diagonally in both directions, checking with a straightedge in several places until it sits flat everywhere.
6. Finish along the grain.A smoothing plane with a fine set, or a sander through the grits. Only now do you work with the grain, because working along it early follows the existing shape rather than correcting it.
7. Flatten one face fully before touching the other.The first flat face is the reference for everything else. Flip and work the second face parallel to it, using a marking gauge run round the edges from the flat face to gauge the thickness.
8. If you have no plane skill, use a router sled.Two straight rails either side of the panel, a bridge running on them carrying the router, and a wide flattening bit. Take shallow passes with plenty of overlap. It is slow and dusty but needs no technique.
9. Accept losing some thickness.Flattening always costs thickness. Take it evenly off both faces rather than all off one, or the panel will move again as the case-hardened surfaces come off unevenly.

Tips

• A panel that arrives cupped may just be reacting to being stored flat on one side. Stand it on edge for a week with air to both faces before you flatten it — a good proportion straighten out on their own.
• Do this work in the room the piece will live in, at the humidity it will live at. A panel flattened in a damp garage will cup again indoors.
• A belt sander is the tool most people reach for and the worst one for this — it digs hollows faster than you can see them. A hand plane or a router sled gives a flat result; a belt sander gives a smooth wavy one.

Common mistakes

• Sanding a panel flat — Abrasives follow the existing surface. A sander smooths a cupped panel into a smooth cupped panel. Flattening needs a rigid tool that bridges the hollows — a plane sole or a router sled.
• Working along the grain from the start — A plane worked with the grain rides the existing contour. Working across and diagonally is what removes high spots and establishes a flat plane.
• Flattening both faces alternately — You chase the shape from side to side and lose thickness for nothing. Get one face genuinely flat, then work the second parallel to it.

Questions people ask