Things You'll Need
40-, 60-, 80- and 120-grit sandpaper for the sander
Dark wood stain
If you've got hardwood floors that are finished too lightly, darkening the floors will take some work, and is a lot more complicated than re-painting a wall a darker color. A finished floor has to be stripped of all existing gloss and stain and taken down to bare wood before being re-stained a darker shade. Then it has to be re-sealed in new gloss. Make sure all that work is going to be worth it before you start. Test your darker stain on a piece of scrap wood and set it in the room to ensure that you like how it looks.
Load your drum sander with 40-grit sandpaper (the roughest grade). Run it over the floor in a diagonal direction to the position of the floorboards, so it begins to grind off the old gloss. Keep the sander moving. Do the whole floor. Vacuum up the dust.
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Re-load the sander with 60-grit paper and sand the floor in the opposite diagonal direction, taking up the rest of the gloss and digging into the existing stain. Vacuum up the dust.
Sand the floor two more times, with increasingly fine sandpaper, with the direction of the floorboards. The final sanding should leave the floor clean and free and all gloss and stain. Vacuum up the dust.
Put on your rubber gloves. Starting in the far corner from the entryway, spread dark stain down over the floor, laying it on thickly and letting it pool. Work in sections as big as you can comfortably reach at once, letting the stain sit on the floor for about a minute and then wiping it up with paper towels. Do the whole floor. Let it dry overnight.
Apply polyurethane gloss starting in the same far corner, using a sponge brush to layer it smoothly with the direction of the floorboards. Do the whole floor. Let it dry overnight.
Buff the dried gloss with 220-grit sandpaper, using quick light strokes to dull the surface of the gloss. Vacuum up the dust. Re-apply a second layer of gloss in the same manner as the first. Let it dry, buff it with 220-grit paper, and apply a third and final coat. Don't sand the last coat, but let it set for at least two days before using the floor.
Wear a dust mask while sanding the floor.
Sanding the floors may spread dust to adjacent rooms; consider hanging plastic sheeting in doorways to prevent the dust from spreading.