How to Read Stainless Steel Finishing: a Sandpaper Grade

The short answer on Stainless Steel Finishing

Here is the governing rule: every finishing pass must be finer than the one before it, and on stainless, P100 is the grit that most jobs pass through. It is the reference point of the sequence — coarse enough that it actually cuts, fine enough that its own marks hide at arm's length. Two scenarios where the rule bends: heavy work with weld cap or mill scale still on the part, where P80 or coarser has to do the removing first, and a part that is already near its final finish, where you start at P240 to P400 instead and never bring P100 back into it. If your job is everyday stainless cleaning, weld work and blending, plan around a P80 to P400 run with P100 in the middle, and confirm the disc or sheet fits your tool before it touches the part. The fundamentals of the material side — how chromium keeps a clean surface in practice, and why contamination shows up later — are covered plainly at stainless steel finishing fundamentals, and a short reference at the abrasives glossary keeps the label terms straight so you and your team read the same sheet.

Two habits turn this from guesswork into a repeatable spec: always state the grit standard you are buying to — FEPA (ISO 848) in most export work — and always confirm the backing and tool speed before the disc or sheet goes on. Everything below is the reasoning behind those two moves.

What P100 does, and what it does not

When someone asks us to read a sandpaper grade for stainless work, P100 is the number that usually sits in the middle of the answer, and it is worth saying what that actually means. A P100 sheet removes material at a speed you can feel in the hand: on mild steel it bites fast and loud, and on stainless it is slower and cleaner, because the alloy work-hardens under the grain. The grit takes out P80 scratches comfortably and leaves its own marks shallow enough that a P240 pass hides them with modest effort. On a plate weld or a brushed joint, P100 is the grit that takes the surface from "ground" to "blended," which is why so many shops treat it as the default mid step in a Stainless Steel Finishing sequence.

What P100 does not do is start a job that has stock on it. Over mill scale, rust stains or a raised weld cap, a P100 sheet loads and glazes instead of cutting — the grain face packs with soft material and the sheet stops taking. In practice we tell customers that P100 arrives as the second step in most stainless work: P60 or P80 removes, P100 cleans up and starts the blend, and P240 to P400 brings the surface to its target. If you only have one grit on the bench, P100 is a defensible choice for general stainless cleanup, but it is not a substitute for the sequence, and it is the wrong grit for polishing, which is P600 and finer territory.

Two concrete numbers on the back of the sheet matter before the grit: the grain and the backing. The grit tells you the size of the cut, the grain tells you how the cut behaves, and the backing tells you how much pressure the grain can take before it glazes or the pad builds heat. Those three are the whole of what you are buying, and the next section works through how they trade against each other on stainless specifically.

The grit ladder for stainless work

The FEPA (ISO 848) scale is what most international buyers quote, and on stainless it reads as a ladder where P100 sits at the center:

P60 🛠️ — strip mill scale, knock down heavy weld cap, open a rough face. Not a finishing grit on stainless.
P80 ⚙️ — fast removal, cap work, coarse stock reduction on a joint.
P100 🔩 — the everyday band for stainless cleanup: weld work, general blending, marking out a P240 finish.
P150 – P240 📏 — blend the ground area into the parent metal, prepare for coating.
P320 – P400 ✅ — cleanup, de-raise, leave a uniform low scratch before paint or clear.
P600 – P1500 ✨ — polish, remove light marks, bring a near-final surface to an even state.

The reference for the scale is at the FEPA grit standard, and two things on that scale deserve a plain note. First, the numbers are inversely related to grain size: P100 is a coarser grain than P240, and P600 is finer still, so "higher number" always means "finer cut." Second, the number is only meaningful when you name the scale, because an older ANSI or mesh number does not line up with a FEPA P100, and that mix-up is how a supplier quietly ships you a coarser grain than you planned. The habit that prevents it is short: name FEPA or ISO 848 in every enquiry, and keep the shop on one scale so the grits on the shelf are comparable.

A quick pair of rules underneath the ladder, both about steps: never jump more than one rung at a time, and never re-visit a coarse grit after you have moved down the ladder. A P80 to P400 jump leaves P80 scratches under the finish, and a P100 pass run back over a P400 surface puts deep marks into it that the next step has to fight. The ladder is one-directional, and the job is done when the last step has erased every scratch from the one before it.

The two scenarios where the P100-center rule does not apply, stated plainly:

Heavy stock removal on thick material. A 3 mm plate with full weld cap needs P60 or P80 to do the removing; P100 only starts once the cap is down. On a belt machine this is where a P80 to P100 belt in aluminium oxide does the opening work, and the paper work only begins after it.
Thin sheet near its final finish. A P100 pass on thin stainless builds heat fast enough to warp the part, and its scratches are too deep for a polish. Start at P240 at most on thin stock, and keep the pad moving instead of pressing into it.

Reading the label, not just the grit

The grit number is the loudest thing on the label, and the habit that saves the most money is reading the rest of it. A typical disc or sheet lists six fields, and each one has a job:

Grain — aluminium oxide, zirconia alumina, silicon carbide or ceramic; it sets how the cut behaves and how long the face stays open.
FEPA grit — the cut size, on the scale you named in the enquiry; a P100 on FEPA is not a "100" on an older mesh scale.
Size — the sheet width or disc diameter, which sets how much of the part you cover per pass.
Arbor or hole — on a disc, the centre hole and any backing plate or hook-and-loop fit; a mismatched hole is how a disc comes loose at speed.
Maximum working speed — the line speed the disc is built for; the tool has to stay below it, confirmed against the model, never assumed.
Colour code — most lines colour-code the grit bands, so a P100 shelf does not quietly become a P240 shelf.

Read all six and the label tells you exactly where the product sits; read one and you are guessing the other five. Two examples of where that guessing bites: a P100 disc on a small-diameter tool with no speed check can run past its line speed on a fast grinder, and an unnamed-scale order can arrive coarser than planned, leaving scratches the sequence was not built to hide. If a label term is unfamiliar, the abrasives glossary defines it, and the sanding disc range shows how the fields are printed on the shapes we supply, from P60 to P5000.

How the abrasive works on stainless: grain, backing, bond

The grit number sizes the cut, but the grain decides how the cut behaves, and on stainless that difference shows up more than on mild steel, because the alloy work-hardens and loads a soft grain. Three parts do the work, and they trade against each other in a predictable way:

The grain — what cuts. Aluminium oxide is the default on stainless: tough, economical, and the grain we spec for most general stainless finishing where you want a predictable, even cut. It loads slowly and is forgiving. Where the job packs the grain — castings, heavy cap, or a job with real material movement — zirconia alumina keeps the face open and the cut honest for longer.
The bond and backing — what holds the grain and sets the pressure ceiling. A soft paper backing that flexes lets the grain dig deeper; a firm, heat-resistant backing keeps the face flat and the cut even. The backing is where most of the choice actually happens, which is why the flap and disc ranges list it on the label.
The grit — what sizes the result. Same grain, different grit, different finish, and the grit is the number you buy to.

The order of thinking matters more than any single choice: match the grain to the material first, then set the grit to the cut, and let the backing follow the tool's speed. Change all three at once and you cannot tell which one moved. A short decision list for the grain, on stainless specifically:

Aluminium oxide ✅ — the all-rounder and the default for cleanup, weld work and general blending on stainless and mild steel alike.
Zirconia alumina ⚙️ — where the job is high-volume and you need the face to stay open through the run; it runs a little hotter, so it suits the firmer backings.
Silicon carbide 🟦 — a harder grain for non-ferrous metals and some composites, where a softer, slicker surface is the target; less common on plain stainless work.
Ceramic ✨ — the premium, longest-wearing option, where it earns its keep on very high-volume work and cost-per-part is the number you manage.

For the grains we stock on discs and flaps and where each fits, the fiberglass-backing flap disc page shows the heat-resistant backing that suits the high-speed end, and the metal fabrication abrasive use notes walk the sequence by material so you can confirm the grain against your alloy before you commit a grit.

One construction point that shows up in every stainless job: the bond has to be strong enough that the grain does not fall out mid-pass, because a patchy face leaves a patchy surface, and on stainless a patchy surface shows under paint worse than it does on mild steel. A sheet that works through its grit life without shedding keeps the scratch band even, which is the whole point of the next step in the sequence.

A worked example: a steel plate weld, P100 on each side

A shop we work with brought us a 3 mm steel plate joint: a butt weld across a panel, cleaned but not finished, heading for a painted coating. The plan that got it there, grit by grit:

P80 on each side of the seam to take the cap back to roughly flush. This is the heavy step, and it wants a firm backing and a moving pad. The goal is flat, not pretty.
P100 across both sides of the joint to even out the ground area and start blending it into the parent metal. This is the pass that sets the surface for everything after it, and on a plate panel it is where the bulk of the work happens.
P240 to blend the transition so there is no visible line where the weld stops and the plate starts. This is the step that decides whether the part reads as "ground" or as a clean panel.
P400 for a final cleanup pass, leaving a uniform low scratch the paint sits evenly on.

The thing the sequence is really doing is keeping every layer of scratch finer than the one before it, and the moment someone skips the P100 or the P240, the coarser scratches survive under the finish and show through. That is the single most common reason a weld looks ground but not finished.

Two variations worth knowing before you plan the job. If the plate still carries mill scale or staining, the sequence starts earlier: P60 or P80 to break through the scale first, and no amount of P100 finesse works on a surface that still has rough black scale on it. And if the panel is thin, drop the pressure, keep the pad moving, and consider starting the blend at P120 instead of P80, because heat on thin sheet warps the part faster than it finishes it.

A short decision list for where each grit belongs on plate work:

Mill scale still on the panel 🧱 — P60 to P80 first; a belt machine makes sense for the opening pass, and the paper work starts only after the scale is gone.
Surface stains on a clean panel 🧊 — P100 removes light staining where the stain is surface-level; anything that has started to pit needs a coarser grit first, and a contaminated stain may also need a proper stainless cleaner before the abrasive work.
Weld cap, no scale 🔩 — P80 to reduce, P100 to blend, exactly as the example above.
Blended, going to paint ✅ — P240 to P400; the finish is set by the last grit in the run, not the first.

For the full sequence by material, the metal fabrication abrasive use notes are the working reference, and the grits and backings we hold for each band are visible in the flap disc range and the sanding disc range, which span grits from P60 to P5000.

P100 and the rust-spot question

This is the section that keeps a stainless finishing job from showing its work three weeks later, and it is about contamination rather than grit. Rust spots on a finished stainless part almost always come from embedded iron: mild-steel particles that migrated into a grit you then ran on stainless, and they sit on the surface where moisture and oxygen do the rest. The chromium in the alloy keeps a clean surface in practice, but a patch of stray iron does not have that protection, so it corrodes first and shows it.

The prevention system is simple, and it is why shops that run stainless keep dedicated grit:

A dedicated P100 for stainless, that never sees a mild-steel job. Shared grits are how iron moves into the grain.
Grit changes between materials, in the coarse-to-fine order: a P80 that did mild steel never comes back to a stainless part, even if it looks clean.
Surface prep before finishing — where staining has already taken hold, the mark is worked out with a coarser grit and a dedicated stainless cleaner first, and only then does the P100 finishing pass run over it.
Keep the finished surface dry and clean until it is coated, because a clean surface and a coated surface behave differently against moisture.

Contamination is also the reason to read the surface before you spec the grit at all: a panel that came back from storage with light spotting needs a P100 cleanup pass after the stain treatment, not a P400 polish over it, because the polish just hides the contamination under a finish that will bloom later. The fundamentals of why the alloy resists, and what breaks that, are at stainless fundamentals, and it is a two-minute read that pays for itself on every finishing enquiry you get.

Deoxidizing, heat tint and scale: where P100 fits and where it does not

Welding puts a heat-tinted band on stainless — the blue, brown and straw discoloration that follows the seam — and most buyers call cleaning that band "deoxidizing," which is roughly right: the work is taking down the thin discolored oxide that the heat put on the surface so the metal reads even. On a small area, P100 is the right grit to skim that layer off without digging deep; the band is thin, and the grit removes it in one pass with the pad moving. That is the honest middle of the deoxidizing range: light tint, light grit, moving pad.

Where P100 stops fitting is the heavy end of the same family. Scale — the rough black layer that forms on a heated steel plate, whether from welding or from a heat treatment — is not a tint, it is a layer, and it comes off with P60 to P80 grits doing real removing, not with a finishing grit skimming. The same split applies to a belt machine: opening passes and descale work are where a P80 belt in aluminium oxide earns its keep on a hot plate panel, and once the face is clean the P100 and finer paper work takes over. If the job is a mix of descale and finish, plan it as two jobs with two grit bands, and confirm the model and its rated speed against the machine before either one starts, because the descale pass wants more line speed than the finish pass does. The stainless steel finishing use notes carry the detail on which band fits which surface condition, and the abrasive belt range lists the sizes we hold for the opening passes.

Speed, dust and safety, in plain terms

Grinding and finishing on stainless is a dust and a flying-chip hazard, and the honest safety message is that compliant use lowers the risk rather than removing it. One number protects most of the job: the disc's maximum working speed. The relationship is straightforward — surface (line) speed rises with both diameter and RPM — so a larger disc on the same tool reaches a higher line speed than a smaller one, which is why the same grinder can be fine with a 100-millimetre disc and too fast for a 180 mm disc. Confirm the tool speed against the disc label before the job, and keep the tool below the printed maximum; if the two do not match, the fix is a smaller disc or a slower tool, not running past the label.

A pre-start check, in order:

The disc's rated speed is above the tool's actual speed, guard fitted.
Eye and face protection on, before the tool starts.
Dust extraction at the work, running, with a filter that suits the material.
The grit being used is dedicated to stainless or clean of any mild-steel carry-over.
The PPE stack and controls are documented, and re-checked at the start of each shift.

The U.S. PPE general requirements are a solid baseline for that stack at OSHA 29 CFR 1910.132, and a practical reading of abrasive wheel setup and use is at CCOHS: Abrasive Wheels.

Dust gets its own line because it is the part shops under-plan. Dry grinding on stainless throws fine metal and abrasive dust that is easy to breathe and easy to track around the shop. Three things do most of the work: a dust extractor with a filter that suits the material, the extractor actually running at the work, and a store bin for the waste instead of a swept corner. On any material that can produce silica, the dust control has to be taken more seriously, because that is the hazard that turns a routine job into a long-term exposure. The dust topic is covered plainly at abrasive dust and airborne particulates.

On the flap side of these jobs, the backing does the heavy work at speed: a steel backing takes the pressure on the removing passes, a fiberglass backing is the pick where the tool runs fast and a plastic backing would flex or overheat, and a plastic backing suits the cooler finishing passes. A disc running hot and flexing is usually the wrong backing for the speed rather than the wrong grit, and the fiberglass-backing flap disc page is where to confirm which backing we hold for your diameter.

Cost, wear and lead time, the numbers that actually matter

On cost, the useful comparison is cost per finished part, not price per sheet or per disc. A P100 sheet that cuts longer and keeps its face open costs more per unit and less per surface, and on a stainless job the difference shows up in how many parts you get between changes, how much re-grinding you do, and how much waste you throw away. For steady volume, that per-part number is the one to build a flap discs wholesale conversation around, because at volume that is where the saving lands.

Wear on stainless has a specific habit worth knowing: a dedicated P100 for stainless lives longer than a P100 shared with mild steel, because the cross-contamination is what glazes the face early. That is another vote for the dedicated-grit system from the contamination section above, and it shows up on the cost side as fewer sheet changes and a more even finish. On high-volume runs, zirconia alumina grains are where the cost-per-part logic pays off, because the longer face life beats a cheaper sheet that changes twice as often.

When the order becomes an OEM or private-label run, the conversation shifts from grit to a spec sheet, and the fields that make or break it are:

Grain and grit — the grain and the FEPA grit, stated on the same scale you shop to.
Standard and sizing — the grit scale named on the label, and the diameter, hole or sheet size matched to the tool.
Backing — chosen for the speed and the job, as the speed section above walks through.
Maximum working speed — confirmed against the model, never assumed, because it has to come from the product.
Label and packing — your logo, part code and any spec sheet you want on the box.
Quantity and lead time — the order quantity and how fast it needs to be on the shelf, which sets whether it ships from stock or gets made to your run.

Lead time has three parts. In-stock grits and common diameters ship fast; off-grid grits or a backing you do not hold in the same run take longer, because the grain, the backing and the label all have to line up. And OEM or private-label work adds time for the label, the packing and any spec sheet you want on the box, with minimum order quantities that are set per item — confirm the MOQ and the lead time with sales before you plan a shelf around them. The request a quotation form is where that conversation starts, and the manufacturing & quality page is where the inspection side of an order is described. One rule on the numbers we share: we list grit, backing and diameter as standard, and we confirm maximum working speed against each model before an order ships, because that number has to come from the product, not from a guess.

When P100 stops cutting

Under-cutting is the quiet complaint: the sheet looks fine, the tool sounds fine, but the part is not moving. On a P100 grit the causes are usually one of five, and each has a different fix:

Pressure, not grit. Pressing into the pad makes the grain load and glaze, which is the opposite of cutting more. If the cut dropped when you leaned in, the fix is lighter pressure and a moving pad, not a coarser grit.
Grain too soft for the material. A grain that is as soft as the workpiece packs and stops; on a material that loads aluminium oxide, move to zirconia alumina and the cut comes back.
Wrong backing for the speed. A pad running hot and flexing is a soft backing on a job that needs a firmer one; the backing limits the cut before the grit does.
Tool speed too low. The abrasive wants to spin; below the speed it was made for, it cuts soft and slow. Check the no-load speed and keep the tool in its design range.
Grit too fine for the job. A P100 on heavy stock is a finishing grit asked to remove; step down to P80 and the removal comes back.

For any of these, the sanding disc range shows the grit and backing to reach for, and the abrasives glossary keeps the terms straight so you and your team are diagnosing the same problem. The thread through all five is that under-cutting is almost always a match problem, not a bad sheet — fix the grain, the backing, the speed or the pressure, and the cut returns.

Common mistakes and how to fix them

The fixes are cheaper than the mistakes. These are the ones that show up most often on stainless work:

Cross-contamination. Running a P100 that did mild steel onto stainless, and getting spots a week later. Fix: dedicated grits, changed between materials, and a stainless cleaner for anything already stained.
Skipped grits. P80 to P400 leaves a band of P80 scratches under the finish; add the P100 and P240 steps and the surface clears.
P100 on scale. Glazing instead of cutting, because the grit is a finishing grit on a removing job; P60 or P80 first, then the P100 cleanup.
Speed mismatch. A disc running past its printed maximum; lower the tool speed or use a smaller diameter, and confirm the rated speed against the model before the job.
Heat on thin sheet. A coarse grit dwelling in one spot warps the part; drop a grit and keep the pad moving rather than pressing in.
No extraction. Dust settling on the finished surface and on the shop floor; run the extractor at the work with the right filter, and bin the waste.

Each of these has a reference point: the stainless finishing notes for the sequence and the contamination rules, the fiberglass-backing flap disc and the flap disc range for the grit and backing to reach for, and the metal fabrication abrasive use notes for the material side. Most of the complaints on stainless are match complaints wearing a grit's name — the grit was fine; the backing, the speed or the sequence was not.

FAQ

Is P100 too coarse for stainless finishing? It depends on where in the sequence you are. P100 is the right grit for weld cleanup and general blending on a plate joint, and it is too coarse for a visible finished face, which wants P400 or finer. Treat P100 as a step in a Stainless Steel Finishing sequence, not as the last one.

What does "P100" actually refer to? The grain size on the FEPA scale, which is ISO 848 in export trade. P100 is not a diameter and not a mesh count, and the "P" prefix is how FEPA marks itself against older scales. Naming the standard in the enquiry is what stops a supplier quoting you on a different scale — see the FEPA grit standard for the reference.

Why do spots appear on a finished stainless part? Embedded iron from cross-contaminated grit is the most common cause, or a surface that was already stained before the finish pass. The fix is the dedicated-grit system, a dedicated stainless cleaner for anything already spotted, and keeping the finished surface dry until it is coated. The why is at stainless basics.

Can I jump from P80 to P400 and skip P100? No. The skipped step leaves its scratches under the finish, and P400 cannot erase a P80 band. Add the P100 and P240 steps and the surface clears.

What grit do I leave on a surface before paint? P400 to P600 is where paint-ready finishes usually sit, and P100 is an earlier step in the run that got there. The last grit in the sequence is the one the surface actually reads as.

Does P100 work on a belt machine? It does, on a belt in aluminium oxide, which is a kinder first pass on a plate panel than paper is. Confirm the model's rated line speed against the machine before the pass, because belt work wants more speed than the same grit on a pad does. The sizes we hold are in the abrasive belt range.

Why does my P100 and another shop's "100" cut differently? Different grains on the same grit, and sometimes different scales entirely. Ask both for the standard (FEPA or ISO 848) and the grain before you compare, and keep your own shop on one scale so the grits on the shelf are comparable.

How do I buy P100 grits in bulk? Send the grain, the grit, the standard, the size, the backing and the quantity to a supplier, and start with a sample against the spec before the full run. The request a quotation form is where that starts, and all abrasive products is where to check what we hold.

Before you buy

A short compliance and purchasing checklist. This is a reminder, not a substitute for your own site rules, and under compliant use with the right protection, the risk of these tools is reduced but not zero.

Confirm your tool's maximum speed is below the abrasive's printed maximum working speed before every use; when the two do not match, the fix is a smaller disc or a slower tool, and the rated speed is confirmed against the model, never assumed.
Check the guard, the backing and any adapter are the correct size for the disc, and fit the guard every time the tool goes on a job.
Wear eye and face protection, and where work goes over the head, a face shield, before the tool starts.
Have dust extraction running at the work with a filter that suits the material, and the right respiratory protection for the dust load; document the PPE stack and re-check it each shift.
Name the grit standard — FEPA or ISO 848 — in every order, so a P100 is a P100 and not an older-scale "100".
Ask for certifications and an inspection report for any bulk or OEM order, and confirm them against the material before the shelf does.
Store abrasives away from moisture and heat, in the packing they came in, and transport them the same way, because a soft sheet glues itself back to its neighbour.
Follow the maker's install notes on tool speed, backing and fitting, and re-check them on a new tool before the first pass.
Keep a record of the PPE and controls you rely on; the U.S. baseline is at OSHA PPE general requirements, and setup guidance is at CCOHS: Abrasive Wheels.
Confirm lead time, minimum order quantity and packing for any bulk or branded run, starting with the quotation form.
For a private-label or OEM order, check the OEM & private-label service page for what we can put on the label, the packing and the spec sheet.

If you tell us the material, the finish you are aiming for and the tool you run, we can help you pre-select the grit and the backing to start from — and if the job is a Stainless Steel Finishing run on a plate joint, we will lay out the P80 to P400 sequence with the grain and backing for each step. Send the three details and we will work it with you on the request a quotation page.