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.