How to Sand and Refinish a Hardwood Floor

Why this matters

A sand-and-finish is judged on one thing: what the floor looks like when the customer stands in the room with the blinds open. Drum chatter, edger swirl, a stop mark where the drum sat down, or a stain that blotched over a missed scratch all show through clear finish, and every coat on top magnifies them. None can be fixed without sanding back through the finish, which costs the job twice and a slice of a floor that can only be sanded so many times in its life. The quality is decided before the first coat, at the sanding.

Grit choice and the machines themselves are 'Sanding Machines and Grit Sequences'. Choosing a finish is 'Hardwood Floor Finishes for Refinishers'. This card is the job from walk-in to final coat.

Step 1: Decide whether the floor can be sanded at all

• Wear layer. Pull a floor register or a threshold and look at the edge of a board. On solid tongue-and-groove, the usable wood is what sits above the tongue; sand into the tongue and the boards split along their edges. On engineered flooring, the maker states how many sandings its wear layer allows. A full sanding commonly removes on the order of 1/32 in, more on a cupped or badly worn floor.
• Fasteners. Face nails and staples have to be set below the surface. A proud nail head tears the drum paper and can throw sparks.
• Moisture. Take readings per 'How to Take and Log Wood Moisture Readings Before Hardwood'. A floor that is cupped because it is wet now will crown if it is sanded flat now and then dries.
• What else is on the floor. Paint on a pre-1978 floor brings in 'Lead-Safe Rules for Floor Work in Pre-1978 Homes' before any sanding. Patches of old resilient flooring or black adhesive in the room are presumed to contain asbestos until tested and go to 'Asbestos in Old Resilient Floors and Adhesive'.

Step 2: Contain the dust and remove the ignition sources

Seal doorways with plastic and zip doors, cover vents and shut off the HVAC so it does not spread dust through the house, and run the sander's dust bag or extraction at every machine.

Fire. Fine wood dust burns readily, and dust from sanding through old finish can heat itself in a full bag. Empty the bags outdoors into a metal container away from the house, every time, never into a trash can in the garage. Solvent-based stains, sealers and finishes give off flammable vapor: before they are opened, shut off and extinguish every pilot light and ignition source in the area, including water heaters and furnaces, and keep them off until the vapor has cleared as the SDS describes. Then relight each per its appliance maker's instructions and confirm it lights and stays lit; one that will not stays off for a qualified gas technician. Rags with oil-based stain or finish on them can ignite as they cure: lay them flat outdoors to dry or keep them in a water-filled, closed metal can.

Power. Big drum sanders run on 240 V, normally through a dryer or range receptacle with the adapter made for it. Tying a machine into a panel is electrician's work, licensed where your state or locality licenses it.

Breathing and hearing. Wear hearing protection on the drum and edger. Where dust or finish vapor cannot be controlled at the source, use respiratory protection under a written program per 29 CFR 1910.134 (applied to construction by 29 CFR 1926.103); two-component finishes with an isocyanate crosslinker need the respirator their SDS names.

Step 3: Run the drum with the grain

Sand the field with the drum sander along the boards. Start about one-third of the way in from one end wall, facing the far wall, so the first passes cover about two-thirds of the room's length. Lower the drum while the machine is already moving, travel to the wall, raise the drum, pull back along the same path lowering it again, and raise it before you stop. Move over by about half the drum width and repeat. When that part of the room is done, turn around and sand the remaining third, overlapping into the first section.

Why the drum never stops while down. A turning drum resting in one place cuts a dip in seconds, and the dip shows through finish as a shadow line across the boards. Lowering and raising on the move is the whole technique.

Why with the grain. Scratches along the grain hide in the wood's own lines. Across the grain they show, so a first cut taken at an angle to flatten a cupped floor is always followed by straight cuts with the grain.

Step 4: Edge, scrape and fill

Run the edger around the perimeter and anywhere the drum cannot reach, with the same grit the drum just used, overlapping the drum's field. Scrape the corners and under toe kicks by hand. Trowel-fill gaps and nail holes with a filler matched to the species and the planned color, after the medium cut so the fine cut takes the excess off.

Then step through the grit sequence: each grit on the drum, then the same grit on the edger, before moving finer.

Step 5: Buff out the marks

Run the buffer with a screen or sanding disc at the final grit over the whole floor, along the grain, concentrating on the band where the edger met the drum. This blends the edger's swirl and the drum's faint lines into one even scratch pattern.

Step 6: Clean, then color and finish to the maker

Vacuum the floor, then the walls, sills and tops of trim, then the floor again. Wipe with the dry tack cloth or microfiber the finish maker names, not a solvent rag unless the maker says so. For stain, test it on a sanded area in a closet or under where a sofa will sit and get the customer's approval on the dried sample. Apply sealer and finish coats at the coverage, dry time and recoat window the maker states, screening between coats when the maker requires it. Once the dust work is over, put the HVAC back on with a fresh filter, uncover the vents, and confirm air moves at each register, run at the temperature and humidity the finish maker gives for curing.

Worked example: a red oak living room

The floor. A living room 14 ft by 18 ft, 3/4 in solid red oak strip, 2-1/4 in face, boards running the 18 ft length. Previous finish worn through to gray wood in the traffic lane from the front door. Slight cupping along 4 ft beside a sliding door.

The checks. At a pulled floor register, a little over 1/4 in of wood above the tongue: room for this sanding and others after it. Moisture readings with a pin meter on the red oak setting were within one point of each other across the room, including the cupped strip by the door. The customer said the cupping had looked the same for years, and the door sill had been resealed the summer before. Stable cupping, so it was safe to sand flat. Two black pet stains near a window were dark through the full board, which sanding does not remove; those two boards were replaced first per 'How to Replace Damaged Boards in a Nail-Down Hardwood Floor'.

The sequence. From the grit card, for a finished floor needing flattening, this job ran 36, 50 and 80 on the drum and edger, then a 100 screen on the buffer (36 to 50 skips 40, 50 to 80 skips 60, never two). The first cut on the cupped 4 ft strip went at a slight angle to the boards to bring the edges down, then straight with the grain.

         wall (boards run left-right)
  +------------------------------------+
  |EE edger band all round, same grit E|
  |E                                  E|
  |E  <==== 2/3 section ====>|<=1/3=> E|
  |E   drum: forward to the  |turn    E|
  |E   left wall, back on    |around, E|
  |E   same path, step over  |overlap E|
  |E   half a drum width     |back in E|
  |E                                  E|
  |EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE|
  +------------------------------------+
   buffer: overlapping passes with the
   grain over the whole floor, slowest
   on the band where edger meets drum

The miss. After the 80 cut, a work light laid on the floor at the window wall showed faint chatter, evenly spaced ripples across the boards, in one pass near the middle. Cause: the paper had loosened on the drum and its seam was lifting. A fresh 80 paper mounted tight, two passes over that strip, and the light showed it gone before the buffer touched it.

Color and finish. The customer approved a medium brown on a closet sample. Stain, then the finish maker's sealer and two coats of a waterborne finish, each at the maker's dry time, screened between coats as its instructions required.

What changes the answer

• A thin engineered wear layer. The coarse first cut that flattened the oak above can go straight through a thin veneer. Where the maker's sandability statement is tight, start finer, take the drum off the floor sooner, and accept that only light wear is removable.
• Maple, birch and other tight-grained species. They take stain unevenly and show sanding scratches more than oak does, so the final grit runs finer and stain is tested on that floor before anyone promises a color, often with the conditioner or water-pop step the stain maker describes.
• Parquet and herringbone. There is no single grain direction, so some passes always cross grain. Those floors are flattened with the drum on a diagonal and brought to a finished scratch with a multidisc or buffer rather than a final straight drum cut.

How to check you got it right

The crew uses a low work light laid on the floor during sanding, because it finds marks while they are still cheap to fix. The customer's test is different, and it is the one the floor is judged by: standing, in the room's normal light.

• Standing at each doorway and at the main window: no chatter, no edger swirl at the perimeter, no stop marks, no fill showing as blotches.
• Sheen: even across the room, no dull patches where the finish was thin, no edges where coats overlapped.
• Trash in the finish: at most a few small specks, none in the traffic lane or at the main sightline.
• Perimeter and corners: sanded and finished into the corners and under the toe kicks, matching the field in color.

Then hand the customer the maker's cure times in writing, for walking in socks, for furniture with pads, and for rugs, which typically come last.

References

• 29 CFR 1910.134, respiratory protection
• The finish and stain makers' application instructions and SDS, which govern coverage, dry and recoat times and ignition controls
• See related: Sanding Machines and Grit Sequences; Hardwood Floor Finishes for Refinishers; How to Take and Log Wood Moisture Readings Before Hardwood; How to Replace Damaged Boards in a Nail-Down Hardwood Floor
• See related: Lead-Safe Rules for Floor Work in Pre-1978 Homes; Asbestos in Old Resilient Floors and Adhesive