Floor finishes are often divided into two cate­gories: penetrating sealers (penetrants) and surface finishes. Penetrating sealers usually contain plant-based oils, such as tung oil or modified lin­seed oil, and soak into wood fiber. In time, they harden to seal and protect the wood. Because penetrating sealers form a hard outer shell, they can be easier to touch up by sanding lightly and adding more sealer if wood becomes scuffed or scratches—touched-up areas won’t be obvious. Wood stains penetrate and don’t really seal.

When they have dried, penetrating sealers are often waxed to make them more durable. But waxed floors are durable only if they’re regularly maintained, which takes time. Thus most floors today are sealed with surface finishes, which don’t require waxing.

Surface finishes, as their name implies, form a tough exterior shell to resist scuffs, scratches, and moisture. The earliest surface finishes were shellac and varnish, which have been largely replaced by oil-based and water-based poly­urethanes. Shellac has poor water resistance and chips easily, and varnishes tend to be strong smelling and slow to dry. Besides, both are extremely flammable. Although surface finishes require less maintenance, their disadvantage is that they can’t be touched up when they become worn, so you must refinish the whole floor. Surface-finish sheens range from matte (little shine) to satin to semigloss to gloss. In general, glossier finishes are harder, more durable, and more water resistant.

As you’ll see, polyurethanes vary greatly in ease of installation, drying time, and durability. Water-based polyurethanes are the stars of the show these days: tough, nonyellowing, relatively mild smelling, and fast drying (2 hours to 6 hours). And brush cleanup is easy with soap and water. Because they contain lower levels of
volatile organic compounds (VOCs), water based polyurethanes are also safer to use. Despite their volatility, oil-based polyurethanes are often applied by professional refinishers because they’re slower to dry and thus allow more time to even out coats. Though solvent based and stronger smelling, oil-based polys are more durable and water resistant, and they turn a handsome amber with age. Let them dry for 24 hours before recoating or walking on them. Moisture-cure polyurethanes are the most durable of the lot; but because they must absorb moisture from the air, they’re temperamental to apply, slow to dry, and best left to professionals.

Finally, there are surface finishes that require highly controlled environments and thus are factory applied to prefinished flooring. This group of finishes may include aluminum-oxide, titanium, or ceramic additives to resist abrasion and may require UV curing, rather than heat cur-

ing. Some of the toughest of these finishes infuse acrylic into the wood cells in a modern version of a penetrating sealer.

and sanding the floor a second time. If you need to fill holes or gaps in the floor, do it before the second sanding.

SCREENING FLOORS

After you’ve drum-sanded and edged the floor with 100-grit sandpaper and vacuumed it well, use a buffer with an abrasive screen to smooth out any remaining marks. Use a 100-grit or 120-grit buffer screen, which is held onto the buffer pad by friction. Because the buffer rotates slowly and the screen is flexible, you can buff
right next to the base of the wall. Start along a wall, moving the buffer from side to side (it rotates in a counterclockwise direction). As you did with the drum sander, overlap passes about one-half the width of the buffer pad. Buffer screens wear out quickly, so replace them when you’ve screened one-third to one-half the floor. Save at least one used screen, so you can fold it and use it to hand-screen the corners where the buffer couldn’t reach.

To achieve an even smoother finish, vacuum the floor and wet-sponge it with clear water the night before screening it (the moisture will raise the grain slightly). The next day, when the wood flooring is dry, screen it smooth. Wetting the wood and then screening it is called popping the grain. Popping is optional, but strongly recom­mended if you’ll be applying a water-based finish. After screening the floor and touching up corners by hand, vacuum the room thoroughly and use clean tack rags to remove dust from any horizon­tal surface. (A tack rag is a slightly sticky cheese­cloth pad that adheres dust.) Finally, dry-mop the floor, wrapping the mop in a clean cloth lightly dampened with the same solvent you used to thin the floor finish.

NOTSHAKEN

finishes to mix them, as you do paint. Shaking will entrap air bubbles and leave blemishes—popped bubbles—when the finish dries. Instead, stir finishes thor­oughly from the bottom of the can. Don’t thin finishes. If stirring doesn’t dissolve the finish "skin” or other solids, strain the finish through a paint strainer.