Sanding is the process of abrading the wood surface with progressively finer grits of sandpaper to create a smooth, uniform surface ready for finishing. While hand planing and scraping can produce superior surfaces (see Chapter 8), sanding is often the most practical approach, especially for:
The grit number indicates the size of the abrasive particles. Higher numbers mean finer particles and a smoother surface.
| Grit | Classification | Use |
|---|---|---|
| 60-80 | Coarse | Aggressive material removal, rough shaping. Avoid on surfaces that will be finished — it creates deep scratches that are very hard to remove. |
| 100-120 | Medium | Removing machine marks, initial smoothing. A common starting grit for surfaces from a planer or jointer. |
| 150 | Medium-fine | General smoothing. The minimum grit for surfaces that will receive stain. |
| 180 | Fine | Standard pre-finish sanding for painted surfaces. |
| 220 | Very fine | Standard pre-finish sanding for clear finishes (oil, varnish, lacquer). The most common “final” grit. |
| 320 | Extra fine | Between coats of finish (scuff sanding). Final sanding before high-build finishes. |
| 400+ | Ultra fine | Polishing between coats, rubbing out finishes. Rarely used on bare wood. |
The fundamental principle of sanding: each successive grit removes the scratches left by the previous grit. This means you must follow a systematic progression.
A typical grit progression for furniture:
120 → 150 → 180 → 220
For surfaces that will receive stain (stain magnifies scratches):
120 → 150 → 180 → 220 (minimum)
For surfaces that will be painted:
120 → 150 → 180 (sufficient — paint fills minor scratches)
If you jump from 80 grit to 220 grit, the 220 grit paper cannot remove the deep 80-grit scratches — it simply polishes the tops of the ridges while the valleys remain. The result is a surface that feels smooth but reveals the coarse scratches under finish. Each grit must be fine enough to remove the previous grit’s scratches, but coarse enough to do so efficiently.
Rule of thumb: Never skip more than one grit step (e.g., 100 → 150 is acceptable, 100 → 220 is not).
Always use a sanding block. Sanding with your fingers creates uneven pressure — your fingertips concentrate force on small areas, creating an undulating surface. A sanding block distributes pressure evenly.
Sanding blocks:
Direction: Sand with the grain — parallel to the wood fibers. Cross-grain scratches are visible under finish and extremely difficult to remove.
Pressure: Use moderate, even pressure. Pressing hard does not make sanding faster — it wears out the sandpaper, creates heat (which can clog the paper), and produces an uneven surface.
Technique:
Random orbit sander: The best power sander for woodworking. The disc spins and orbits simultaneously, creating a random scratch pattern that is virtually invisible under finish.
Technique:
Belt sander: Aggressive, for rapid stock removal and flattening. Not for finish sanding — leaves visible directional scratches. Always sand with the grain.
Detail sander (mouse sander): A triangular-pad sander for corners and tight areas. Useful but leaves a distinctive scratch pattern — hand-sand the areas afterward with the grain.
The goal is a uniformly smooth, flat surface. The danger is “dishing” — sanding a depression into the surface.
Prevention:
End grain is harder and denser than face grain. It needs more sanding to reach the same smoothness.
Technique: Sand end grain one grit finer than the face grain. If you sand face grain to 220, sand end grain to 320. This compensates for the density difference and ensures even stain absorption.
When wood gets wet, the fibers swell and stand up, creating a rough, fuzzy surface. This is called “raised grain.” If you apply a water-based finish to unsanded wood, the grain will raise and the finish will feel rough.
The solution: Intentionally raise the grain before finishing.
Tip: If using a water-based stain, raise the grain before staining. If using an oil-based stain followed by a water-based topcoat, you may still want to raise the grain before staining, as the water in the topcoat can partially raise it.
As covered in Chapter 8, a card scraper can replace much of the sanding process:
The ideal workflow for many projects: plane → scrape → light sand at 220 → finish.
Before applying finish, inspect the surface thoroughly. Finish does not hide flaws — it magnifies them.
Hold a light source (a bright LED or halogen work light) at a very low angle to the surface — almost parallel to it. This “raking light” casts shadows from every scratch, dent, glue spot, and imperfection that would be invisible under normal lighting.
Walk the light across the surface, looking at each area. Mark any flaws with a pencil and address them before finishing.
Wipe the surface with mineral spirits (white spirit). The wet surface simulates how the wood will look under a clear finish. Check for:
Mineral spirits evaporate without residue, so this test does not affect the subsequent finishing.
Coarse grits (60-80) remove material fast but leave deep scratches that take a long time to remove with finer grits. Use coarse grits only when necessary — and spend less time with them, more time with medium and fine grits.
Already covered above. Each grit must erase the previous grit’s scratches.
The most common mistake. Each grit must be used long enough to fully replace the previous grit’s scratch pattern across the entire surface. How do you know when you are done with a grit? Mark the surface lightly with a pencil, then sand. When all pencil marks are gone, the entire surface has been sanded.
This wears the paper out faster, generates heat (which softens and clogs the paper), and creates an uneven surface. Let the abrasive do the work.
Cross-grain scratches are visible under finish and very difficult to remove. Always sand with the grain as your final step, even if you used a random orbit sander previously.
Sanding beyond what is needed (e.g., sanding to 400 grit for a project that will receive a brushed varnish) wastes time and can actually reduce finish adhesion. The finish needs some “tooth” (microscopic roughness) to grip.
After final sanding, all dust must be removed before applying finish:
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