Once you know your belt’s size, two choices remain: how coarse it is and what it is made of. The grit decides how fast the belt cuts and how fine a surface it leaves. The abrasive grain decides what it cuts well and how long it lasts. Neither affects whether the belt fits your machine; that is only width and length.
Reading the grit number.
The grit number is printed on the back of the belt. Lower numbers mean larger grains and a coarser, faster cut; higher numbers mean finer grains and a smoother surface. Two scales are in use:
- FEPA, with a “P” in front (P80, P120). The European scale, now used by most abrasive makers worldwide.
- CAMI, a plain number (80, 120). The older North American scale.
Up to about 220 the two scales are nearly the same, so P120 and CAMI 120 cut alike. Above that they drift apart: matching CAMI 600 takes about P1000 to P1200. FEPA grades also hold their grains to a tighter size range, which gives a more even scratch pattern. Fine Woodworking’s comparison explains the difference in detail.
Which grit for which job.
| Grit | Typical use on a belt sander |
|---|---|
| P24–P40 | Heavy removal: old paint and varnish, levelling rough boards, shaping metal on a grinder |
| P60–P80 | General removal and flattening; taking out saw marks |
| P100–P120 | Smoothing the scratches from coarser grits |
| P150–P180 | Final sanding of bare wood before a finish |
| P220 and finer | Between coats of finish, and fine work on metal |
Work through the grits in steps without skipping more than one, so each grit only has to remove the scratches of the one before. Jumping from P40 to P180 leaves deep scratches the fine belt can’t reach. A belt sander is mostly a removal tool; many woodworkers finish with a random-orbit sander or by hand.
The abrasive grain.
- Aluminium oxide. The all-round grain for wood and general metalwork. Inexpensive, and the usual choice for handheld belt sanders.
- Zirconia alumina. Tougher grains that keep breaking into fresh sharp edges under pressure. Good for heavy removal, hardwoods and steel; lasts longer than aluminium oxide when you press hard.
- Ceramic. The hardest-wearing option for heavy grinding of steel and stainless steel, as on belt grinders. It works best with real pressure; with a light touch it can glaze.
- Silicon carbide. Very sharp but brittle. Suits non-ferrous metals, stone, glass and sanding between coats of finish.
Coating and backing.
Open or closed coat. A closed-coat belt is fully covered in grains and cuts fast on hard materials. An open-coat belt leaves gaps between the grains so dust and paint have somewhere to go; it clogs less on softwood, resinous wood and old paint.
Backing. Belts for belt sanders and grinders have cloth backings. Lighter, more flexible cloth follows curves and contact wheels; heavier cloth stands up to heavy grinding and high tension. Check whether your manual names a backing for your machine.
Direction. If the belt has an arrow on the back, its joint is made to run one way. Fit it so the arrow points in the direction the belt travels.
Find your machine’s belt size ↗︎Sources
- Mark Schofield, “True Grit”, Fine Woodworking #176, March/April 2005.