Wood & laminate
Refinishing a hardwood floor: grits, sandpaper, time and finish
A worn wood floor can look new again for a fraction of the cost of replacing it. The job is mostly sanding, and the sanding is mostly arithmetic: area, grits, abrasive life and time.
Recoat or refinish?
Not every worn floor needs sanding to bare wood. A screen and recoat scuffs the existing finish with a fine screen and adds a fresh coat. It suits a floor whose finish is dull and lightly scratched but not worn through to the wood. It takes a day and costs a fraction of a full refinish. The test: where the floor is most worn, drop a little water on it. If it beads, the finish is intact and a recoat will work. If it soaks in and darkens the wood, the finish is gone there and the floor needs sanding.
A full refinish sands through the old finish to bare wood, then applies stain if wanted and new finish. It removes scratches, stains that have not gone deep, and the grey of worn areas.
How many times can a floor be sanded?
A solid 3/4 inch floor has about 1/4 inch of wood above the tongue, and a full sand removes about 1/32 inch, so in principle it can be sanded many times; in practice, six to ten over its life. An engineered floor depends on its wear layer: a 4 mm layer once or twice, a 2 mm layer perhaps not at all. Pull a floor register and look at the edge of a board to see how much wood remains above the tongue.
The grit sequence
| Floor | Grits | Passes |
|---|---|---|
| Screen and recoat | 120 screen | one |
| New unfinished floor | 60, 80, 100 | three |
| Sound floor, full refinish | 36, 60, 80, 100 | four |
| Thick finish, cupping or deep scratches | 24, 36, 60, 80, 100 | five |
Start with the finest grit that removes the finish in a reasonable time; a coarser start removes more wood and leaves deeper scratches to sand out. Never skip more than one grit, or the next grit cannot remove the previous one’s scratches. The edger, which does the strip along the walls the big sander cannot reach, follows the same sequence.
Abrasives and time
Each grit uses its own belts for the drum sander and discs for the edger. A belt sands roughly 200 sq ft at coarse grits and 300 sq ft at fine grits before it stops cutting. Buying too few is the commonest mistake; most rental shops take back unused abrasives.
A 550 sq ft living area, full refinish
A sound floor with 100 ft of wall, sanded at 36, 60, 80 and 100 grit with a rented drum sander and edger, by one person.
- Edge strip: 100 ft × 0.5 ft = 50 sq ft for the edger; field 500 sq ft for the drum sander
- Abrasives: 9 belts (3 at 36 grit, 2 each at 60, 80 and 100) and 8 edger discs
- Sanding: 500 sq ft ÷ 200 sq ft an hour × 4 passes = 10 h; edging 6 h 40 min; vacuuming 2 h 12 min
- Total: 18 h 52 min of work, about 3 days, plus drying time for the finish
The floor sanding and refinishing calculator does this from your floor’s area, perimeter and condition. The perimeter calculator gives the wall length for the edger.
Finish
A refinish takes a sealer coat and two or three coats of finish. Oil-based polyurethane covers about 500 sq ft per gallon per coat and dries overnight between coats; water-based covers about 450 sq ft and dries in a few hours. For the 550 sq ft floor, one sealer and three coats of water-based finish is four coats at about 1.2 gallons each, call it five gallons. The finish calculator on our sister site Paints Calculator works it out from the same area.
Stain adds a coat and a day, and it shows every sanding mark, so the final sanding must be thorough. Water-pop the floor (wipe it with a damp cloth to raise the grain) before staining for an even colour.
Safety
Sanding dust is fine and, in a pile, can self-ignite when it contains finish or stain. Empty the sander’s bag outdoors into a metal container, never leave it full overnight, and lay out stain and oil rags flat to dry or keep them in water in a sealed metal can. Wear a respirator rated for fine dust and ventilate the room while the finish cures. Old finish from before 1978 may contain lead; test it first.
DIY or professional?
A drum sander is unforgiving: hold it still for a moment and it digs a dip into the floor that takes another pass to remove. Professionals use belt sanders with dust extraction and multi-disc finishers, and work two to three times faster than a first-timer with rental machines. For a large or valuable floor, the cost of a professional refinish compares well with the rental, abrasives, finish and time. The installer labour cost calculator prices the professional job, and the guide to installer pricing explains the figures. If the floor is past refinishing, the hardwood calculator counts the replacement, and the comparison of wood floors covers the alternatives.