80 Grit vs 100 Grit Sandpaper for Floors

Use 100 grit as your final drum pass for water-based polyurethane or any stain work. Use 80 grit for oil-based clear polyurethane. Never skip either grit in the NWFA sequence — each one erases the scratch pattern of the previous. Run the 10-minute water-wipe test before you crack open the finish can. The rest of this guide explains the physics of why skip-grits show under finish and tells you exactly when each grit is the right call.

The 10-minute water-wipe test (do this before you finish)

Scratches that look invisible on a dry sanded floor under shop lights show up under finish under sun-angle light at 6 PM. The water-wipe test simulates the finish without committing.

1. After your last sanding pass (drum + edger + buffer), vacuum the floor.
2. Wet a clean rag with plain water. Wring it nearly dry.
3. Wipe a 4×4 ft patch of floor. The water darkens the wood the way polyurethane will.
4. Wait 5 minutes for the water to start drying.
5. Look at the patch from a low angle — kneel down, look across the floor toward a window or a flashlight at ankle height. The raking light reveals scratch patterns the dry floor hides.

If you see uniform fine scratches in one direction, you’re done. If you see crosshatched scratches, swirl marks, or visible coarser scratches from a previous grit, you’re not done — go back to the grit that left them and sand them out before you commit to finish.

This test takes 10 minutes. Pulling polyurethane off a finished floor because you skipped it takes two days.

Picks for the actual decision

Your finish	Stop at	Why	Where to buy
Oil-based polyurethane, clear (no stain)	80 grit drum + 120 grit buffer screen	Oil poly is thick and forgiving; hides 80-grit scratches	VA 80 grit, 8×19.5” 50-pack
Water-based polyurethane, clear (no stain)	100 grit drum + 120 grit buffer screen	Water-based is transparent; 80-grit scratches read under it	VA 100 grit, 8×19.5” 50-pack
Any stain (oil or water-based topcoat)	100 grit drum + 150 grit buffer screen	Stain blotches on uneven scratch depth; finer = more uniform absorption	VA 100 grit + Norton 100 closed-coat search
Acid-cured / Swedish finish	60–80 grit drum	Finish bonds chemically; finer surface = worse adhesion (counterintuitive but documented)	VA 60 or 80 grit
Engineered floor (any finish)	100 grit only, single pass, light pressure	See pre-rental check — or screen-and-recoat instead	VA 100 grit

The Norton link carries the same affiliate tag as ours — we earn the same small commission whichever brand you pick. It’s the better sheet at 100 grit when stain quality matters.

Quantity: budget 2–3 sheets per grit for a 250 sq ft room (a 50-pack of each covers many floors). For per-square-foot math with species multipliers, run the sheet calculator.

Why skipping a grit shows under finish — the physics

Each grit cuts a valley pattern in the wood. 36-grit valleys are about 0.45 mm deep. 60-grit valleys are 0.25 mm. 80-grit is 0.20 mm. 100-grit is 0.15 mm. Each finer grit removes the peaks of the previous valleys but only flattens — it can’t reach the bottom of the previous valley.

Skip a grit (say, 36 → 80) and the 80-grit pass flattens the floor visually but the 36-grit valleys are still there underneath, untouched at the bottom. On a dry floor under overhead light: invisible. Wet the floor with finish: light reflects off the valley bottoms at a different angle than the flat field, and the valleys read as parallel dark streaks running in the direction of the original 36-grit pass.

This is also why staining a skip-grit floor looks blotchy — stain pools deeper in the leftover valleys and reads darker there.

There is no shortcut. Sand back to bare wood and start the sequence over, or live with the result.

80-grit as a final pass

Scratch depth: ~0.20 mm. Visible under raking light on a dry floor. Hidden under oil-based polyurethane (thick, fills the scratches). Visible under water-based polyurethane (transparent, telegraphs surface texture).

When 80-grit is enough:

• Oil-based clear polyurethane finish (the thick finish coat hides the scratch pattern)
• Dark stain that will hide any scratch pattern below it (espresso, ebony, dark walnut)
• Budget jobs where time matters more than the finest-finish quality
• A 120-grit buffer screen will follow (the buffer blends 80-grit scratches into the final surface)

When 80-grit is not enough:

• Water-based polyurethane (transparency reveals every scratch)
• Light stains (natural, golden oak, whitewash) where scratch contrast shows
• Premium jobs where the customer will look at the floor under raking light from a low window

100-grit as a final pass

Scratch depth: ~0.15 mm. Subtle even under raking light. Invisible under any finish.

When 100-grit is the right call:

• Water-based polyurethane (the transparent finish needs the finer scratch pattern)
• Any stain work (uniform scratch absorption = uniform color development)
• Premium residential finishes
• Light stains where scratch visibility matters most

When 100-grit is overkill:

• Oil-based clear poly on darker species (the finish hides scratches at 80 grit already)
• Heavily damaged floors where additional sanding time isn’t justified
• Acid-cured finishes (finer surface actually reduces adhesion)

NWFA grit sequence with the 80 vs 100 decision baked in

Finish type	Drum sequence	Buffer screen
Oil-based clear poly	36 → 60 → 80	120 grit
Water-based clear poly	36 → 60 → 80 → 100	120 grit
Stain (any finish over)	36 → 60 → 80 → 100	150 grit
Engineered hardwood	100 only (or screen-and-recoat)	120 or 150 grit
Acid-cured / Swedish	36 → 60 → 80	none (skip the buffer)

Never skip more than one grit. Each grit erases the previous; skip one and the scratches show through.

For the full grit progression reference, see our sandpaper grit chart.

Common mistakes with grit selection

Using 80-grit for everything

Cost-effective but produces visibly inferior results on light stains and water-based finishes. The scratch patterns are subtle but present, especially under side lighting from a window in the morning or evening.

Not accounting for stain color

Dark stains hide scratches. Light stains reveal them. Match grit to the planned finish — know your stain before you finalize the sanding sequence. Don’t “decide on stain after the floor is sanded.”

Skipping the 60-grit pass

Some DIYers go 36 → 80 to save time. Doesn’t work. The 80-grit can’t cut down the 36-grit valleys. See the skip-grit physics section above.

Going past 100 on the drum

Anything finer than 100 grit on the drum closes the wood grain (burnishing) and blocks stain absorption. Save fine grit for the buffer.

Assuming water-based poly can skip finer grits

Water-based polyurethane requires finer surface preparation than oil-based. Using 80 grit with water-based without a buffer screen pass means:

• Visible scratch patterns showing through the finish
• Potential adhesion issues in high-traffic areas
• Uneven stain acceptance if you’re staining

Quick decision summary

Choose 80-grit final drum pass if:

• Oil-based polyurethane (clear)
• Dark or medium stain planned
• Budget-constrained

Choose 100-grit final drum pass if:

• Water-based polyurethane
• Light or natural stain
• Premium finish expected
• You want the floor to look right under any light condition

Either way: Always follow with a buffer screen pass. 120 grit for clear poly, 150 grit if staining. The buffer is what blends the field with the edger work.

Run the water-wipe test before you finish. Ten minutes there saves two days of redo.

For brand picks at each grit (VA for cut, Norton for finish when staining), see our hardwood floor refinishing guide. For the full step-by-step process, see how to sand hardwood floors.

Run the water-wipe test before you crack open the polyurethane can.

Matt.