Sanding Machines and Grit Sequences
Why this matters
A sanded floor looks fine bare. The defects show up after stain or the first coat: edger swirls ringing the room, chatter marks across the boards, a band of coarse scratches where a crew jumped from 36 grit straight to 80. By then the only fix is to sand again. The machine sets what kind of mark you leave; the grit sequence decides whether each pass removes the last pass's scratch. This card covers both. The sanding procedure itself belongs to How to Sand and Refinish a Hardwood Floor, and abrasive minerals and backings to Common Abrasives + Sanding Reference.
Before the machine is plugged in
Is there paint on this floor? A painted border around an old rug line, a painted floor upstairs, or paint on the base being sanded against: in a pre-1978 home, machine sanding a painted surface not tested lead-free is a prohibited practice under the EPA RRP rule (40 CFR Part 745, Subpart E) unless the machine has HEPA exhaust control, and it is a lead exposure for the crew either way. Stop and work it through Lead-Safe Rules for Floor Work in Pre-1978 Homes. Black adhesive or old resilient tile under a carpet is the asbestos question, which is never sanded: see Asbestos in Old Resilient Floors and Adhesive.
Power. Big machines commonly run on 240 V. Plug into a receptacle rated for the machine with the cord set made for it, or a generator sized for it. Connecting at the panel is electrical work: it goes to someone qualified, licensed where your state or locality requires it, never to the sanding crew with the dead front off.
Dust. Wood dust is a respiratory hazard (IARC classifies it as a human carcinogen), and some species are sensitizers. Run the machines' dust collection, keep doors to the rest of the house closed and masked, and wear a respirator under a written program per 29 CFR 1910.134 (applied to construction by 29 CFR 1926.103). Fine sanding dust, especially dust carrying old finish, can self-heat in a full bag: empty bags outside into a metal container with a lid at every break and at the end of the day, and never leave a full bag in the house overnight or in the truck cab.
The machines and the mark each one leaves
Drum sander. A rotating drum with paper clamped around it. It cuts fast and flat, and it punishes hesitation: a drum that stops turning against the floor, or drops without the machine moving, cuts a dip across the boards in a fraction of a second. Lower it while walking, raise it before stopping, every time. Chatter (a regular washboard pattern across the floor) points to the machine: a worn drive belt, out-of-round drum, loose wheel or bad paper seam.
Belt sander. The big-machine alternative with an endless belt. Its longer contact patch rides small humps instead of following them, so it chatters less and is more forgiving of a pause. It still gouges if dropped on a stationary floor.
Edger. A rotary disc that reaches the last few inches along walls and into areas the big machine cannot. It leaves circular scratches across the grain and, if tipped, scallops (shallow dished arcs). Edger marks are the most common visible defect on a refinished floor because they cross the grain, and stain finds them.
Multidisc (planetary) sander. Three or more discs on a rotating plate. It flattens, removes edger swirl, and blends the perimeter into the field. On most refinishes it does the final pass.
Buffer with a screen or sanding disc. A rotary floor machine used to screen between coats, blend, or abrade an existing finish for a recoat. It is a finishing tool, not a flattening tool.
Corners and toe kicks get a hand scraper and a hand block, sharpened and pulled with the grain.
The grit rule
Each grit leaves scratches of a certain depth. The next grit has to remove those scratches and leave its own shallower ones. Jump too far and the finer grit polishes the tops of the old scratches without reaching their bottoms; they look gone bare and reappear under stain or finish.
The rule: never skip more than one grit size between passes. NWFA sanding guidance teaches the same limit. Count the skip on your supplier's ladder. Many floor-paper lines stock 16, 20, 24, 36, 40, 50, 60, 80, 100 and 120. On that ladder, 36 to 50 skips one step (40) and is allowed; 36 to 60 skips two (40 and 50) and is not. If your supplier's ladder has other steps, count on theirs.
The edger follows the big machine through the same grits, so the perimeter has had the same scratch removal as the field before the multidisc blends them.
The grit card
GRIT SEQUENCE CARD
ladder: 16 20 24 36 40 50 60 80 100 120
(never skip more than one step)
A. HEAVY FINISH, OR CUPPING NOW STABLE
big machine 24 at an angle -> 36 -> 50
edger 24 -> 36 -> 50
multidisc 80
B. NORMAL REFINISH, FINISH WORN BUT SOUND
big machine 36 -> 50
edger 36 -> 50
multidisc 80
C. NEW SITE-FINISHED FLOOR, FLATTEN OVERWOOD
big machine 40 at an angle -> 60
edger 40 -> 60
multidisc 80
ALL: screen at 100 only if the finish maker
calls for it; finer final grit = lighter stain
Each sequence obeys the rule: on card A, 24 to 36 is adjacent, 36 to 50 skips 40, 50 to 80 skips 60; on card B, the same two steps from 36; on card C, 40 to 60 skips 50 and 60 to 80 is adjacent.
The angled first cut. On a cupped, crowned or uneven floor, the first pass runs at a shallow angle to the boards (commonly 7 to 15 degrees) so the drum bridges board edges instead of following each one; every later pass runs with the grain to remove the angled scratch. A cupped floor is sanded only after the moisture problem behind the cupping is fixed and the boards have stabilized; sand a floor that is still cupped from moisture and the edges you cut off become a crown when it dries. See related: The Hardwood That Cupped the First Summer.
Final grit and stain. Coarser final scratch opens the grain and takes stain darker; finer closes it and takes stain lighter. That is why the final grit is fixed before the stain sample is made, on the same floor, and not changed afterwards. Water-popping (raising the grain with water before stain) deepens color further, per the stain maker.
Worked example: a heavy finish that will not cut
A 400 sq ft red oak living room with three coats of an old oil-modified finish, worn through in the traffic lane, no cupping, no paint. The plan was card B.
Trial. The crew runs 36 grit on a 3 ft strip along the wall where the sofa sat. The paper loads with gummy finish within a few feet and the strip still shows finish in the low spots of every board. That is card A, not card B: the floor needs 24 to cut the finish.
Sequence. 24 at an angle across the room, then 36 with the grain, then 50 with the grain, edger at 24, 36 and 50 behind each big-machine pass, then the multidisc at 80. On the ladder, 24 to 36 skips nothing, 36 to 50 skips 40, 50 to 80 skips 60. No step skips two.
Pencil check. Before each grit, the crew draws light pencil lines across a few boards in the field and along the edged perimeter. A pass is done when every line is gone, which shows that pass reached below the previous scratch everywhere, including at the walls.
The near-miss. At 50 the crew is tempted to go straight to the 100 screen to save time. On the ladder, 50 to 100 skips 60 and 80: two steps. Under a medium-brown stain the 50-grit scratch would have shown in every board. The multidisc at 80 stays in.
What would change it. An engineered floor instead of solid: measure the wear layer at a register opening or a threshold edge and read the floor maker's statement on whether it can be sanded at all and how many times. A thin wear layer may allow only a screen and recoat, and a 24-grit start can go through it. A prefinished floor with an aluminum oxide finish and beveled edges: that finish resists abrasives and removing the bevels takes much of the wear layer; it is a different decision, often a recoat with the maker's adhesion system instead of a sand.
Tracing a mark back to its cause
| Mark seen after stain or first coat | What made it | The fix next time |
|---|---|---|
| Even washboard ripples across the boards | Machine: drive belt, drum, wheels or paper seam | Service the machine; the grit sequence was not the cause |
| One dip across several boards | Drum dropped or stopped on a stationary floor | Lower and raise only while moving |
| Rings of swirl within a foot or two of the walls | Edger scratch not removed: edger skipped a grit, or no multidisc blend | Edger through every grit, multidisc at the final grit |
| Straight coarse scratches with the grain across the whole field | A skipped grit on the big machine | Re-sand from the grit after the last one fully completed |
| Angled scratches visible across the boards | Angled first cut never removed by a with-grain pass | Every angled pass gets at least one full with-grain pass after it |
A mark found after stain is cheaper to fix than one found after the first coat of finish, which is the reason to stain a test area at the wall before staining the room.
Reading a sanded floor before finish
Get down low and look across the floor with a work light on the floor. Coarse scratches, edger swirls and chatter catch the light. Then judge it the way the customer will: standing, in normal light. A raking light finds defects no one will see; the standing look decides whether the floor is ready for color.
References
- NWFA (National Wood Flooring Association) sanding and finishing guidelines
- EPA 40 CFR 745 Subpart E (RRP), prohibited practices for painted surfaces in pre-1978 target housing
- OSHA 29 CFR 1910.134, Respiratory Protection, applied to construction by 29 CFR 1926.103
- Sanding machine, abrasive and finish manufacturers' instructions
- See related: Common Abrasives + Sanding Reference; How to Sand and Refinish a Hardwood Floor; Hardwood Floor Finishes for Refinishers