Table of Contents
You can spend $600 on a random-orbit sander and still produce a surface that looks like it was chewed by a beaver. The machine is rarely the problem. The paper is. Every scratch in a board is a message from the last abrasive that touched it, and the sequence you run decides whether the surface comes out flat, uniform, and ready for finish or keeps telegraphing old scratches through every coat you spray.
There is no single best grit, only a correct grit for the stage you are in. The same P120 that levels a glued-up panel will ruin a veneered door in seconds. The same P400 that polishes a film finish will load and glaze if you run it on raw oak. What follows is the reasoning behind grit selection: how the numbers are defined, how the American and European scales diverge, how abrasive minerals and coatings behave, and how to build progressions that stop at the right place. Grit progressions get most of the attention, but the mineral, coat, and backing behind the number matter just as much.
What the Grit Number Actually Measures
A grit rating describes abrasive particle size, not cutting speed or finish quality. CAMI 220 paper carries particles that average about 66 microns across; CAMI 80 averages roughly 192 microns. That three-to-one difference explains why an 80-grit scratch stays visible under raking light while a 220-grit scratch disappears the moment finish touches it.
The relationship is not linear, and that is where most sequences go wrong. Moving from 80 to 120 removes about 76 microns of particle diameter. Moving from 400 to 600 removes a fraction of that, even though the printed numbers travel 200 points. Build your steps on particle size ratios instead of arithmetic and the progressions start making sense.
Particle size also predicts how a paper behaves at the surface. Above roughly 100 microns, abrasives cut deep and fast, leaving a scratch pattern that takes real work to erase. Between 40 and 100 microns, they shape and refine. Below 40 microns the paper mostly polishes the peaks left by its predecessor, which is why fine grits remove shockingly little material no matter how hard you lean on the sander.
CAMI vs FEPA: Why P400 Is Not 400 Grit
The United States grades coated abrasives on the CAMI scale, printed as a bare number. Europe uses the FEPA scale, always with a P prefix, defined under ISO 6344 with tighter tolerances on particle distribution. Up through about P220 the two systems track closely enough to treat as equivalent. FEPA P220 averages 68 microns; CAMI 220 averages about 66.
Past P220 they diverge sharply. FEPA P400 averages 35 microns, which lands much closer to CAMI 320 at 36 microns than to CAMI 400 at 24 microns. FEPA P800 averages about 22 microns, near CAMI 400. FEPA P1200, at 15.3 microns, is roughly CAMI 600. A P400 disc and a 400-grit disc bought from different stores are two different papers, and running them back to back creates a hidden gap in your sequence.
The practical rule is simple. Below P220, treat CAMI and FEPA as interchangeable. Above P220, read the label for the prefix and convert before choosing the next step. FEPA’s tighter distribution also makes P-graded fine papers more consistent sheet to sheet, which matters when you are chasing a uniform scratch pattern across a large panel.
The Progressive Sequence and the 50 Percent Rule
Each successive grit should be roughly 50 percent finer than the one before, which keeps every step within reach of the previous scratch. In practice: 80, 120, 180, 240, 320, 400. Starting finer is often smarter than starting coarse and hoping. If the surface is already milled reasonably flat, start at 120 and skip heavy stock removal entirely. There is no prize for beginning at 60 grit and spending an hour undoing it.
Bare Wood: Stop at 180 or 220
For stained work, stop at 180. For clear film finishes, 220 is the practical ceiling on most hardwoods. Sanding bare wood past 220 burnishes the fibers and closes the pores; stain then cannot penetrate evenly, and the result is blotchy color that no number of additional coats will fix. Softwoods raise grain when wet, so a light 320 pass after a distilled-water wipe knocks the fuzz down without over-polishing. Where tearout or mill marks resist sanding, a card scraper removes them without driving abrasives any deeper into the fibers.
Between Coats: 320 to 600
Between finish coats, the paper’s job changes from leveling wood to leveling finish. Use the finest grit that removes nibs and dust without cutting through the coat: usually 320 to 400 on solvent finishes, 400 to 600 on thin waterborne finishes. One or two light passes by hand with a flat block beats a hard pass with a machine, especially near edges.
Verification With a Pencil Grid
Mark the surface with a light pencil crosshatch before sanding a stage. Sand until the lines disappear, then stop. The grid tells you when the previous grit’s scratches are gone far more reliably than your eyes, especially on light woods where scratch patterns hide. A raking light at a low angle, or a wipe with mineral spirits, reveals any leftovers.
Choosing the Mineral: Garnet, Aluminum Oxide, Zirconia, Ceramic, Silicon Carbide
Garnet is a natural, friable abrasive that dulls quickly but leaves a forgiving scratch. It suits hand sanding softwoods and light finish prep, and it stays cheap.
Aluminum oxide is the workhorse: tough, fracture-resistant, and available in heat-treated versions that hold an edge longer. It handles everything from 80-grit stock removal to 400-grit finish work on wood and is what most quality paper uses by default.
Zirconia alumina is the belt and heavy-removal mineral. The grains self-sharpen as they fracture, keeping a fresh edge exposed to the work. On a belt sander it can outlast aluminum oxide two or three to one on coarse work, although it is rarely offered above 220 grit.
Ceramic is the premium choice for hard, dense stock. Microcrystalline ceramic grains stay sharp far longer than conventional abrasives, cut faster with less pressure, and run cooler, which reduces glazing on hard maple, hickory, and any job where the sander works hard. The price per sheet is higher, but the cost per board foot removed is often lower.
Silicon carbide is sharp and brittle. It is the standard for wet/dry paper, between-coat scuffing, and sanding finishes and metals. Let it cut with light pressure; too much force fractures the grains early and turns a fine paper into a dull one.
Open Coat vs Closed Coat
A closed coat covers 100 percent of the backing with abrasive. It cuts aggressively and leaves the most uniform scratch, which is why most fine finishing papers are closed coat. The tradeoff: softwoods, resinous pine, and painted surfaces clog the spaces between grains and load the paper within minutes.
An open coat covers 50 to 70 percent of the backing and leaves gaps for swarf to escape. It resists loading, runs cooler, and lasts longer on gummy material, at the cost of a slightly slower cut and a slightly less uniform scratch. On pine, fir, and finish removal, open coat is usually the difference between one sheet and four.
Stearated Paper and Finish Work
Stearated paper carries a zinc stearate soap coating that acts as a dry lubricant, and it is the single most useful upgrade for finishing work. The stearate keeps resin and finish dust from welding to the grains, which is where loading and heat come from in the first place. On cured finishes and between coats, a stearated P400 can outlast plain paper several times over while generating less heat and fewer pigtails.
There is one caution. Stearate can leave a residue that interferes with waterborne finishes and some epoxies, causing fish-eye or poor adhesion. If your topcoat is water-based, either use non-stearated paper or wipe the surface with a compatible cleaner before the next coat. On bare wood headed for oil or solvent finishes, stearated paper is not a problem.
Paper, Film, and Mesh Backings
Backing determines how flat the abrasive stays under pressure. Paper backings run from A, the lightest, to F, the heaviest. Buy C-weight or heavier for disc and sheet work above 220 grit; light backings deform under load, tip the grains, and leave deeper scratches than the grit should produce. Belts use E and F weights, which can take the tension and tracking loads without stretching.
Film backings use a thin polyester sheet instead of paper. The thickness is consistent to within microns, so the abrasive stays in a single plane and the scratch pattern stays uniform. Film lasts longer, resists humidity, and does not shed fibers. It costs more, but for fine sanding on a flat panel it is the best backing available.
Mesh abrasives are an open weave that lets dust pass through to the extractor. They cut fast, stay cool, load far less, and many can be washed and reused. The compromise is edge behavior: mesh conforms to contours, so it rounds over edges on flat work unless it runs over an interface pad. For work in tight corners or on profiles, that conformability is a feature rather than a flaw.
Random-Orbit Scratch Patterns and Swirl Marks
A random-orbit sander leaves a pattern based on orbit diameter and pad speed, not a fixed direction. A 5-inch sander with a 3/32-inch orbit leaves fine, tight scratches suited to finishing; a 6-inch sander with a 3/16-inch orbit cuts faster and leaves a broader pattern that needs finer grits to erase. Changing pads between grits, or at least brushing dust off the hook-and-loop, prevents a stray coarse grain from a previous disc from gouging the surface.
Swirl marks from a random-orbit sander usually trace to three causes: skipping a grit, worn paper, or too much pressure at the edges where the pad overhangs. Keep the sander flat on the surface, let the machine’s weight do the work, and move slowly enough that the pad makes overlapping passes. Moving too fast leaves a visible rosette that only shows up under finish, and by then the fix is a full re-sand.
Sanding Between Coats Without Burn-Through
Between coats, the goal is to level the finish, not the wood. Use a fine grit with a light touch and a flat block, and stay away from edges, corners, and profiles where the film is thinnest. Edges cut through first and almost always at the worst time, after the final coat, when a repair means sanding back and respraying a whole panel.
On veneer and thin finish films, the risk is higher. A random-orbit sander can chew through a coat in one careless pass, especially on an arris where the paper overhangs the pad. Hand sanding with a block gives you control. If you must use a machine, keep it moving and never let the pad stop in place.
Veneer, Plywood Edges, and Thin Stock
Commercial veneer runs about 1/42 inch thick, roughly 0.6 mm, and it does not forgive mistakes. Sand with 180 to 220 only, use a block or an interface pad, and never run a random-orbit sander on a veneered surface unless you know exactly how thin the face is. Cross-grain scratches on veneer show up permanently under finish, and a burn-through means a new panel.
On plywood edges, the alternating plies and glue lines cut at different rates. A light 120 pass with a hard block levels the plies without digging trenches into the soft grain between them. If the edges will be visible, a chamfer or a solid edge banding is a better answer than aggressive sanding.
Dust Extraction, Paper Life, and Personal Protection
Dust sitting between the abrasive and the surface does two bad things: it reduces the cut rate and it re-scratches the wood. Good extraction keeps the grain exposed and can extend paper life by half or more. CFM sizing matters as much as the sander’s dust port design, because a sander moves a high volume of fine dust and a choked hose simply stops collecting.
Sanding dust is the finest, most respirable dust in the shop, and it stays airborne for hours after the sander stops. A tight-fitting half-mask respirator with P100 filters is the right standard for long sanding sessions. N95 masks are a compromise at best against fine hardwood dust. If the sander has a bag, empty it outdoors, and never blow dust off a surface with compressed air in an enclosed shop.
Frequently Asked Questions
Can I skip from 120 grit straight to 220?
No. The 120-grit scratches are roughly twice the size of the 220-grit particles trying to remove them, so the finer paper only polishes the peaks and leaves the valleys intact. The surface looks acceptable in the shop and turns into a map of old scratches under the first coat of finish. Step through 180 first.
What grit should I stop at before applying stain?
Stop at 180 on most hardwoods. Going to 220 or beyond burnishes the surface, closes the pores, and causes stain to absorb unevenly. If you already sanded too fine, wipe the surface with a damp cloth and re-sand at 180 to reopen the grain.
Is FEPA P400 the same as 400 grit?
No. P400 averages about 35 microns, which is close to CAMI 320; CAMI 400 averages about 24 microns. Above P220 the two scales diverge, so mixing papers from both systems without converting can leave a hidden step in your sequence.
How do I know when a sheet is worn out?
Watch the cut rate rather than the clock. When the paper stops removing pencil lines with the same effort, when the surface starts feeling hot, or when dust stops clearing from under the pad, the grains are dull. Stearated and ceramic papers hold an edge longer, but all paper is a consumable; a worn sheet costs more time than a fresh one.
Can I sand bare wood and finish coats with the same paper?
Keep them separate. Paper used on bare wood picks up resin and grit that will contaminate a finish scuffing pass, and stearated finish paper can leave soap residue on raw wood headed for a waterborne topcoat. Label or physically separate the two sets.
Does sanding past 220 really hurt stain absorption?
Yes, on most species. Burnished fibers resist penetration, and the result is blotchy color, especially on cherry, maple, and pine. If a project needs a very smooth surface before stain, use a sanding sealer or a wood conditioner rather than finer paper.