What a grit actually leaves behind
Sanding does not smooth wood. It cuts a field of parallel scratches into it, and every grit in the sequence exists to replace the scratches of the previous grit with shallower ones. The surface you finish on is not smooth either; it is scratched finely enough that the finish fills it and the eye stops resolving it. That is the whole mechanism, and once you hold it in your head the rest of the subject stops being folklore.
It follows that the depth of the scratch, not the number on the paper, is what the next grit has to deal with. An 80 grit scratch is deep. A 120 grit abrasive can remove it because it is one conventional step away. A 180 grit abrasive removes very little per pass by design, so asking it to clear 80 grit scratches means many more passes, generated heat, and a loaded disc, and in practice most people give up before the scratches are gone and only discover them under the first coat of finish.
Why the step is roughly 1.5x
The conventional sequence steps by about half again each time: 80, 120, 180, 220. Sometimes it is tighter, 80, 100, 120, 150, 180, which is what production shops with dedicated machines tend to run because each pass is quick. Sometimes it is looser than 1.5 and people get away with it on softwoods where the scratch collapses under the next pass anyway.
The 1.5 figure is a practical convention rather than a physical constant, which is why it is an editable field on this page rather than a fixed rule. Set it to 1.3 and you will get more grits with less work in each. Set it to 2.0 and you will get the sequence most people believe they are running when they are actually leaving scratches behind.
How far to go, and the case for stopping earlier
| Finishing at | Typical use | Why not go finer |
|---|---|---|
| 150 | Under paint and primer | Paint needs tooth. Polishing under it wastes the pass. |
| 180 | Under most film finishes on hardwood | Beyond this the film hides the difference. |
| 220 | Softwood, and under stain | Too fine burnishes softwood and stain then goes blotchy. |
| 320 | Between finish coats | You are levelling a film, not shaping wood. |
| 400 and up | Hand-rubbed and oiled surfaces | Diminishing returns on anything under a film. |
Sanding finer than the finish can resolve is the most common way to waste an afternoon. It is also actively harmful with stain, because a burnished surface will not take pigment evenly and the result is blotching that no amount of extra sanding fixes.
What this estimate cannot know
The minutes here come from an area, a rate you set, and a multiplier for the first pass. None of those know your machine, your species, or how flat the stock was when you started. Reclaimed oak with embedded grit is a different job from a freshly planed poplar panel of the same size. Contours, mouldings and inside corners are hand work and run several times the flat-panel rate. Treat the totals as a way of comparing two plans against each other rather than as a schedule.
The skip penalty deserves the same scepticism. Setting it to 2.5 is a defensible estimate of what a grit costs when it has to do two grits of work, and it puts the crossover in about the right place, but it is not a measured coefficient. What is reliable is the shape of the result: past a certain point, removing a step from the sequence adds time rather than saving it, and the finish shows the difference either way. When the surface is ready, the finish coverage calculator sizes the material and the finishing schedule calculator works out how many days it will actually take.
Questions people ask
Can I skip from 80 straight to 180?
You can, and the surface will usually tell on you. The 180 abrasive is designed to remove very little material per pass, so clearing an 80 grit scratch pattern with it takes many passes, heats the surface and loads the disc. Most people stop before the scratches are actually gone, and the first coat of finish makes them visible because the finish darkens the scratch and the light catches it. If you are going to leave a step out, leave out one in the middle of the run where the surface is already flat, not the one immediately after the coarsest pass, which is where the deepest scratches are.
How do I know when a grit is finished?
The practical test is that the scratch pattern is uniform and no scratch from the previous grit remains. Wiping the surface with mineral spirits or a damp rag darkens it briefly and makes leftover coarse scratches visible in a way that a dry surface does not. Raking a light across the surface at a low angle does the same thing without wetting it. Both are worth doing before moving to the next grit, because finding a missed scratch at 220 costs one pass and finding it under the first coat of finish costs the whole panel.
Is 220 always enough before finishing?
For most film finishes on hardwood, 180 is enough and 220 is comfortable. Going finer than that under a film finish is generally wasted effort because the film fills what remains. Under stain, going too fine is worse than wasted: a burnished surface refuses pigment unevenly and the result is blotching. Under paint, stop at 150 because paint needs some tooth to grip. Where fine grits earn their place is on surfaces that will be oiled or hand-rubbed rather than coated, because there the wood itself is what you are looking at.
Does the sanding sequence change for hand sanding?
The sequence does not, but the time does, and by a lot. A random orbit sander removes material through many small overlapping arcs and does it fast. A block by hand removes far less per minute and follows the grain, which is why hand sanding is generally reserved for the last grit or two, for details a machine cannot reach, and for removing the swirl a random orbit leaves. If your job is entirely by hand, raise the minutes per square foot several times over the default and expect the coarse pass in particular to dominate the total.
Why does my sanded surface look fine dry and terrible once finished?
Because finish changes the optics. Dry sanded wood scatters light and hides shallow scratches; a film finish fills them, raises the refractive contrast and turns them into visible lines. Stain does something worse by depositing pigment preferentially in the scratch. This is why the mineral spirits wipe is worth the two minutes it costs — it previews what the finish will do while you can still fix it. Once the first coat is on, correcting a scratch means cutting back through the finish and starting the affected area again.