Sandpaper grit
20 grits with the European P grade, the approximate American equivalent, and the grain size that defines both.
"400 grit" is two different papers
Two scales are in use. FEPA numbers carry a P prefix and are the European standard; CAMI numbers have no prefix and are American. Up to about 180 they agree closely enough to ignore. Above that they separate, and by the fine end they are a full step apart: P400 is roughly CAMI 320.
The consequence is practical. A finishing schedule written in an American book — 220, 320, 400 — followed with European paper gives a coarser result at every step than the author intended, and the surface shows it under finish. The P is the only thing on the packet that distinguishes them, and it is easy to miss.
The grain size is the honest number
Both scales are defined by an average particle diameter, and that figure is comparable across everything. P400 is 35 µm whoever made it. Where a chart and a packet disagree, the micrometre figure is the one to trust.
The full chart
| FEPA | CAMI | Average grain | Agree? | Typically used for |
|---|---|---|---|---|
| P40 | 36 | 425 µm | yes | Stripping paint and levelling rough timber |
| P60 | 60 | 269 µm | yes | Heavy shaping and taking off old finish |
| P80 | 80 | 201 µm | yes | The usual first grit on bare timber |
| P100 | 100 | 162 µm | yes | General sanding on softwood, and the point at which the surface starts to look intentional rather than attacked |
| P120 | 120 | 125 µm | yes | The workhorse grit for wood before finishing |
| P150 | 150 | 100 µm | yes | Between the shaping grits and the finishing ones |
| P180 | 180 | 82 µm | yes | The last grit where P and CAMI still mean nearly the same thing |
| P220 | 220 | 68 µm | yes | The last grit most woodwork needs before paint or stain, and the point where the two scales still agree closely enough to substitute one for the other without noticing |
| P240 | 220 | 58.5 µm | yes | The first grit of the finishing range rather than the preparation range, and the first where the two scales visibly part company: P240 falls between CAMI 220 and 240 and matches neither |
| P280 | 240 | 52.2 µm | no | Fine finishing on hardwood, and between coats of varnish |
| P320 | 280 | 46.2 µm | no | De-nibbing between coats |
| P400 | 320 | 35 µm | no | The clearest case of the mismatch |
| P500 | 360 | 30.2 µm | no | Fine finishing and light wet sanding |
| P600 | 400 | 25.8 µm | no | Wet sanding paint and polishing metal |
| P800 | 500 | 21.8 µm | no | Between coats on a high-gloss finish, and the start of the polishing range rather than the sanding one |
| P1000 | 600 | 18.3 µm | no | Wet sanding clear coat before compounding |
| P1200 | 800 | 15.3 µm | no | Removing orange peel from paint |
| P1500 | 1000 | 12.6 µm | no | Final wet sanding before machine polishing on automotive paint |
| P2000 | 1200 | 10.3 µm | no | Refining a polished surface |
| P2500 | 1500 | 8.4 µm | no | The fine end of what is generally sold |
Do not skip more than one step
Each grit exists to remove the scratches left by the one before it. Jump from 80 straight to 220 and the finer paper polishes the coarse scratches rather than removing them — the surface feels smooth, looks acceptable dry, and shows every mark once finish goes on and the grain darkens. The usual sequence doubles roughly each time: 80, 120, 180, 220, and stop unless the finish needs more.
Going too fine has its own failure. Above about P220 on softwood the grain closes and stain stops penetrating evenly, which produces a blotchy result that no amount of further sanding fixes.
What each range is actually for
The numbers only mean something against a job. Coarse papers remove material and leave scratches that the next grade has to remove; fine papers remove almost nothing and exist to erase the grade before them.
| Range | What it does | When |
|---|---|---|
| 40–60 | Removes material fast, and leaves gouges | Stripping paint, levelling a badly uneven surface, shaping |
| 80–120 | Takes out the coarse scratches | The first grade on most bare timber, and where sanding usually starts |
| 150–180 | Prepares for a finish | The last grade before stain or paint on most work |
| 220–320 | Smooths without cutting | Between coats of finish, and the point past which bare wood stops absorbing stain evenly |
| 400 and up | Polishes what is already flat | Wet sanding a finish, metal, plastic — not bare timber |
Sanding bare wood past about 220 is usually a mistake rather than an improvement. Burnishing the fibres closes the surface, and a closed surface takes stain patchily — the parts that took most polish take least colour.
Wet or dry, and why the backing matters
Grit is only half the specification. The other half is what carries it: paper backings tear under water, cloth backings survive it, and film backings stay flat enough for finishing work where paper would telegraph its own texture.
"Wet and dry" paper is silicon carbide on a waterproof backing, and the water is not a convenience — it floats away the swarf that would otherwise clog the abrasive and turn the sheet into a burnisher. The same grade used dry loads up in seconds on a finish and lasts a long time wet.
Aluminium oxide is the general-purpose grain for wood and it fractures as it wears, exposing fresh edges. Silicon carbide is harder and sharper and it does not, which is why it starts faster and finishes sooner. Neither is better; they wear differently.