The bevel gets all the attention. That's the problem.
You grind it, set your angle, hone through the progression, feel for the wire edge. The back? You flatten it once, probably at the start, probably not well enough, and then you go back to the bevel every session like it's the only variable.
Most sharpening frustrations are back problems. The chisel that won't get sharp no matter how long you spend on the stone. The plane iron that cuts clean on one side and tears on the other. The edge that seems sharp until you use it. These aren't bevel problems. They're back problems dressed up as bevel problems because the bevel is where you're working.
This is fixable. But only after you stop misdiagnosing it.
What Flat Means
The back of a cutting tool needs to be flat across the full width, corner to corner, in the zone that actually matters: the first 15–25mm behind the cutting edge. A hollow in the middle of a chisel back, 50mm from the edge, isn't your problem. A hollow within 20mm of the edge is.
Most new tools come from the factory with some degree of hollow. This isn't careless manufacturing. It's a side effect of grinding flat on a rotating wheel — the wheel's arc transfers to the surface. What you get is a back that's slightly concave along the length, sometimes across the width too.
Here's why this matters. A hollow back contacts the stone at two points: the edge and the heel, with a valley between. When you apply pressure, you're honing the heel. Not the edge. The wire edge you're trying to form keeps rolling over instead of breaking clean. You can spend thirty minutes on a fine stone and make the problem worse.
Reading the Back
Three tests, in order of speed versus certainty.
Raking light. Hold the blade back-side up and tilt it until light rakes across the surface at a low angle. A flat back shows an even sheen. A hollow shows a dark band down the length. Takes five seconds.
Pencil grid. Draw a grid across the full width and down the length of the back, corners included. Hone on your flattest stone for thirty seconds. Where the pencil marks survive, that surface isn't contacting the stone. A flat back loses all marks evenly. A hollow loses them at the corners first, leaves pencil in the center. Takes two minutes and tells you exactly what you're dealing with.
Shavings test. Take a full-width shaving from a straight-grained reference piece. A blade with a flat back cuts evenly across the width. A blade with a hollow back cuts lighter in the center and leaves a faint ridge in the shaving.
Use raking light for quick checks, pencil grid when something isn't working. The shavings test settles arguments.
Fixing It
You need a flat stone. That constraint doesn't move. A hollow stone compounds the problem. Check yours before you start. A waterstone that cups can be lapped flat on a diamond plate or on plate glass with silicon carbide slurry.
For a manufacturing hollow, start at 220–400 grit. For a wear hollow on a tool you've been maintaining, 800–1,000 is usually enough. Draw the pencil grid first. Hone using the whole surface of the stone, keeping the blade flat throughout. Check after 20–30 strokes.
The goal is contact across the full width behind the edge and into the corners. This is the slow part. Do it right once and every subsequent session is fast. Rush it and you'll be chasing this same problem for the life of the tool.
Once the coarse stone shows even contact, move through your progression. Each grit refines the scratches from the one before — you're not re-establishing geometry at each stage, just refining the surface. On the finishing stone, the back should show an even bright reflection across the full width. Gray or hazy corners mean the geometry problem isn't solved.
Maintenance
A properly flattened back stays flat in normal use. What keeps it that way is a session habit: after honing the bevel, two or three light passes on the back at the fine stone. This removes the wire edge and maintains the edge transition. Not re-lapping — finishing.
Every five to eight sessions, do a quick raking light check. If it's still flat, a few passes at medium and fine and you're done. If the hollow is returning, or if you're getting uneven wire edge formation (breaks on one side but not the other), go back to the coarse stone.
One note on plane irons specifically. The chip breaker seats against the back of the iron. If that back is hollow, the chip breaker can't make a clean seal and shavings will jam in the gap. If a plane jams frequently and you've checked everything else, look at the chip breaker seat. It's probably this.
The Order
Back first, bevel second. Not because the back takes more work — once it's established, it usually doesn't — but because bevel work produces a wire edge that has to come off the back. Find a back problem after honing the bevel and you've added a step you didn't need.
Before every sharpening session: what angle is this tool at, is the bevel still at that angle, is the back flat.
That third question should come first if you want to be honest about where sessions actually fail.