Sandpaper grit numbers confuse almost everyone at first, because the number gets smaller as the abrasive gets coarser and the sequence reverses depending on whether you are sanding bare wood, between coats of finish or a piece of metal. A grit chart solves that only if you know which column you are reading.
This is a practical explanation of what the numbers mean, how the scales relate to each other, and which sequence to use for the work most people actually do in a small shop.
What the number measures
The number is a measure of particle size, and it is defined differently by the coatings-close-coated abrasive standard and the bonded standard used for metalwork. In the coated abrasive standard that covers sandpaper and sanding discs, a P80 particle is roughly 190 microns across, P120 is around 125 microns, P180 around 82 microns, P240 around 58 microns and P400 around 35 microns.
The practical reading is simple: halving the particle size roughly means doubling the number. That is why jumping from 80 to 180 in one step leaves visible scratches; the two grits are far apart in particle size.
The two scales people mix up
There is a CAMI scale used mostly in North America and a P scale used in Europe and in most modern abrasive manufacturing. The numbers do not match: CAMI 100 is close to P120, CAMI 220 is close to P220, and the two scales diverge more at the coarse end.
If a chart lists one column, check which scale it uses before you compare grits across brands. A disc sold as 120 on one scale and 120 on the other will not cut the same.
The sequence for bare wood
For most furniture work the sequence is 80, 120, 180, 220. Start at 80 only if the surface needs flattening; starting at 80 on a surface that is already flat removes material you will spend the rest of the project trying to preserve.
Do not skip more than one grit at a time in the middle of the sequence. Jumping from 80 to 180 is the single most common cause of swirl marks that only appear once the finish goes on.
Stop at 220 for most film finishes and at 180 for stain, because finer abrasives burnish the surface and reduce how much stain the wood will absorb.
The sequence for finish sanding
Between coats, use 320 to 400 grit, and use it lightly. The purpose is to knock down dust nibs and flatten the surface, not to remove the coat. Wet sanding with 600 to 1000 grit is the step that produces a glass-smooth film on a cured finish.
If the finish is still soft, sanding will clog the paper instantly. Wait longer rather than pressing harder.
Metal, plastic and paint
Aluminium and steel run from 120 to 400 for general clean-up, and up to 1000 or beyond for a polished surface. Plastic scratches visibly at coarse grits and should start at 320 or finer. Paint removal is the one job where 40 to 60 grit is appropriate, and only on a surface you intend to refinish completely.
Why the backing matters as much as the grit
A cloth-backed belt resists heat and stays together; a paper-backed sheet is cheaper and tears at the edges. Film-backed abrasives hold grit more evenly and last longer, which is why they dominate the finer grits where consistency matters.
Open-coat abrasives shed dust and resist clogging on softwoods and paint. Closed-coat abrasives cut faster and clog sooner, which makes them the wrong choice for resinous timber.
Building a usable grit stock
Most small shops need five grits, not fifteen: 80, 120, 180, 220 and 320. Buy those in quantity and add 400 or 600 only when a specific finish demands it. Storing abrasives flat and dry matters more than brand loyalty, because a curled sheet wastes more material than a cheap one.
When you compare products, a sander grit chart is worth keeping on the wall next to the sander, mainly to stop the temptation to skip a step when a project is running late.
Common mistakes that show up in the finish
Pressing hard does not sand faster; it dulls the abrasive and heats the surface. Let the paper cut and move the sander steadily. Moving too fast leaves swirls, and moving in one direction only makes those swirls line up into visible scratches.
Not cleaning between grits is the other frequent error. Dust left from a coarse grit keeps cutting when you move to a finer one, so a quick vacuum or tack cloth between steps saves a whole extra pass later.
When to throw abrasive away
Abrasive is consumable and the cost of replacing it is small compared with the time lost to a dull sheet. If a sheet needs pressure to cut, if the surface is glazing, or if the grit rubs off on your fingers, it is finished.
Power sanders change the sequence
Random orbital sanders behave differently from hand sanding because the pad rotates and orbits at the same time, which means the scratch pattern never lines up in one direction. That makes them forgiving: a random orbital sander at 180 grit produces a surface that looks and behaves like a hand-sanded 220, and it is far harder to gouge a surface with one than with a belt sander.
Belt sanders are the opposite. They cut fast, in a straight line, and they will remove a millimetre of material in seconds while you are still deciding whether to press harder. On a flat panel that needs flattening they are unbeatable; on a finished surface they are a liability.
Matching the grit to the machine
A random orbital sander usually starts at 80 and finishes at 220. Because the orbit blends the scratches, skipping from 120 straight to 220 is usually safe. A belt sander is normally run at 60 to 120 and then finished by hand or with an orbital, because its scratch pattern runs across the grain and shows through any film finish.
If you sand a tabletop with a belt sander and then apply oil, the cross-grain scratches appear as pale streaks that no further sanding will hide. That is the reason most shops keep a belt sander for stock removal and an orbital for everything visible.
Reading the chart in the shop
The most useful thing a grit chart does is stop you from guessing after a mistake. If a surface has 80 grit scratches and you are at 220, you need to go back to 120 and work forward rather than sanding harder at 220. The chart makes that obvious, and it saves both material and time.
Photocopy the chart and tape it to the wall. A chart in a drawer is a chart nobody reads.
