Seven posts about the hand plane and every one of them has been about process. The iron, the set, the pass, the skew, the edge, the grain. This one is about the destination.
You're not making a flat board. You're making a surface that holds a finish, tells the truth about the wood, and takes light the way the species wants to be seen. That sounds like mysticism. It's not. There are two tests. When both pass, you're done.
The raking light test
One lamp. Low angle. Near-parallel to the surface. Not overhead, not ambient room light. One source, raking across.
You're looking for four things.
Plane tracks first. A slight ridge where overlapping passes didn't blend. The raking light makes these visible when they're too shallow to feel. One more pass, oriented slightly off your previous angle, blends the transition.
Tearout patches next. Small pits, lifted fibers. If you corrected for grain direction and they're still there, light passes with a tight mouth will get most of them. The card scraper handles the rest.
Mill marks. If the board came off a planer, the raking light shows the scallop pattern at the cutter spacing. The hand plane removes these cleanly. Light passes, long strokes. You're finishing, not flattening.
High spots last. A slight convex rise in a wide panel, a glue joint that's fractionally proud. Winding sticks found the big movement before you started. The raking light finds the residual. Two or three targeted passes.
When the light shows even, uninterrupted shadow across the whole face, that's the visual pass.
The fingertip test
Run your fingertip with the grain. Lightly. Not pressure. Contact.
A surface ready for finish feels slightly cool and uniformly smooth. What you don't want: warmth or drag. Warmth means raised fibers, either from a dull pass or from tearout the light showed as a shadow. Drag means a ridge the light didn't catch. The fingertip takes five seconds and tells you exactly what a coat of finish will find.
When the fingertip agrees with the raking light, you're done with the plane.
The plane as a finish tool
Here's what most people get backwards. The hand plane isn't just for dimensioning. In its final passes, a sharp iron severs fibers cleanly at the surface. Sandpaper abrades them. The surface structure is fractionally torn rather than cut. Under a penetrating finish or a clear film, a hand-planed surface has a depth and clarity that a sanded surface doesn't match.
Practical consequence: if the surface passes both tests, think hard about whether you need to sand at all. On a hardwood with a film finish, a final pass at tight mouth followed by one light hand-sand at 220 is often all the prep you need. That hand-sand isn't correcting the planed surface. It's scuffing for adhesion.
On species that blotch (cherry, soft maple, alder), a shellac wash coat after planing and before any stain matters more than whether you stopped at 180 or 220. The planed surface is a better platform for that wash coat. Fewer broken fibers means more even absorption.
When to stop
Woodworkers get this wrong in both directions, reliably.
Stop too early and the surface looks fine under ambient light. Then you put a coat of finish on it and every plane track, every mill mark, every ridge lights up like a road map. Finish doesn't hide surface problems. It defines them.
Stop too late and you've passed both tests but taken four more passes because it seemed like it could be better. Now you're chasing a theoretical perfect that the finish will never reveal. Worse, you've risked taking the surface out of flat while hunting a high spot that was already within tolerance.
The tests are the gate. When both pass, put the plane down.
The arc in one sentence
The iron is why this surface is possible. The surface is why the iron was worth sharpening. Every post between them was about staying calibrated: set up the tool, read the wood, respond to what the shavings tell you, stop when the tests pass.
That's the whole practice. Not just the hand plane.