Sanding, Grit And Why Order Matters

Sanding looks like smoothing but is really a controlled scratching process. Every grade of abrasive leaves its own pattern, and the job is to replace each pattern with a finer one.

Grit numbers describe particle size

The number on sandpaper refers to the size of the abrasive particles, with higher numbers meaning smaller particles and shallower scratches.

Coarse grits remove material quickly because each particle cuts deeply. That speed is also why they leave marks that must be dealt with afterward.

Fine grits remove very little material and mostly refine what is already there, which is why starting fine on rough wood accomplishes almost nothing.

Skipping grades leaves work undone

Each grade needs to remove the scratches left by the one before it. A large jump means the finer paper is polishing the peaks while the deeper valleys remain.

The surface can feel smooth to a hand while still holding those deeper scratches, because fingertips read the high points rather than the depth.

Moving through the grades in reasonable steps is faster overall than jumping ahead, because reworking a surface after finish has revealed the marks costs far more time.

Finish reveals what sanding hid

Stain and clear finishes both enter scratches, and deeper scratches hold more pigment or reflect light differently, so they appear darker or duller than the surrounding wood.

Cross-grain scratches show most, because wood fibers running one way make a mark running the other way conspicuous.

Wiping the surface with mineral spirits or a damp cloth before finishing simulates the effect temporarily and shows problems while they are still easy to fix.

Dust between grades is not optional

Loose grit from a coarse pass sitting on the surface will be dragged along by the next, finer paper and cut scratches the finer grade should not be making.

Vacuuming or wiping between grades removes that contamination, and it also stops the abrasive from clogging, which is what makes paper stop cutting.

Knowing when to stop

Sanding wood past a certain fineness begins to burnish the surface, closing the pores so that stain cannot penetrate evenly.

The result is blotchy color, and it is a common outcome of assuming that finer is always better on a piece that is going to be stained.

Where the surface will be painted, the finish itself fills fine scratches, so the useful stopping point is considerably earlier than it would be under a clear coat.