Grit progression

The rule behind the planner

Every grit removes the scratch left by the one before it. That only works if the size gap is small enough for the finer paper to cut through the coarser groove. In practice, a step is safe when the next grit is no more than about double the micron size of the previous one — which is why the standard sequence roughly doubles at the coarse end and closes up at the fine end.

Skipping a step does not save time. The finer sheet spends its life polishing the ridges between the coarse scratches, and the deep grooves stay there, invisible until finish goes on and the light catches them.

There is a second trap at the other end: going too fine. Sanding bare wood past 220 burnishes the surface, closing the grain so stain sits on top instead of soaking in. The planner will happily give you 2500 grit because polishing metal genuinely needs it — but it is the wrong answer for a pine shelf.

Wet or dry?

Wet sanding uses water or a lubricant to stop the paper clogging, and it is the right choice above roughly 400 grit. Dust that would load a dry sheet turns into a slurry and washes away, so the abrasive keeps cutting and the surface stays cool. Below 400, dry sanding is faster and the paper can take it.

Wet sanding needs paper designed for it — the backing is waterproof and the bond is resin. Ordinary paper falls apart. It also needs a drop of washing-up liquid in the water, which breaks the surface tension so the slurry carries away instead of balling up.

How to know a step is done

Change the direction of sanding between grits. Sand the next grit across the previous scratch pattern for several passes, then return to the grain. When you can no longer see the earlier scratches, that step has done its job. This is the only test that works reliably, and it costs nothing.

Raking light helps. A torch held almost flat to the surface shows every scratch; ordinary overhead light hides them until the finish goes on and then reveals all of them at once.

The mistakes that show up in the finish

The first is skipping the middle. Going from 80 to 220 because it seems quicker leaves 80-grit grooves that the finer paper only polishes the tops of. They stay invisible until the first coat of finish, and then every one of them shows under raking light.

The second is over-sanding bare wood. Past 220 the fibres are burnished flat rather than cut, the surface stops absorbing, and stain sits on top with a patchy, uneven tone. If a stain has gone blotchy, the grit sequence is the first thing to check, not the stain.

The third is worn paper. Glazed, shiny paper is not cutting any more — it is rubbing. Pressing harder makes it worse: it burns the surface and loads the sheet. Change the paper. Abrasive is cheaper than the workpiece.

Questions people ask