A hardwood floor needs some kind of sealer, or the wood can be easily damaged by impacts, humidity, spills and general foot traffic. Before polyurethane varnish became the default finish for flooring professionals, available options included wax, penetrating oil and shellac. Many homeowners prefer the natural look of oil, but wax and shellac aren't used much anymore, and because they aren't compatible with durable modern finishes, they are best avoided.
Sand and Stain
You should always sand the floor before sealing, even if you're installing new, unfinished flooring boards, and it's important to seal as soon as possible after sanding, whether you use penetrating tung oil or a film finish, such as oil- or water-based polyurethane. Sanding opens the grain and allows the sealer to penetrate, but the open grain leaves the wood vulnerable to moisture in the air, which can swell and discolor it. Any stain you use to change the color of the wood must penetrate, so it goes on before the sealer. Stains don't do the same job as sealers, though. They consist mostly of pigments and solvents that quickly evaporate, so it's important to seal as soon as the stain dries.
Sealing With Oil
Tack the Floor With a Damp Rag
Remove all sanding dust and other debris that may have accumulated.
Wipe the Oil With a Rag
Start in the corner farthest from the door. Pour oil on the floor and wipe it into a 2-by-2-foot section of the floor, moving your rag in the direction of the floorboards. Wipe off the excess oil with another rag before moving on to another section.
Apply at Least Two Coats
Allow the first application of oil to dry for 24 to 48 hours, then apply a second. Repeat the process if necessary.
Sealing With Polyurethane
Use a Pad or Flooring Applicator
A flooring applicator is a foam cylinder on the end of a long handle; it's weighted so you can drag it to spread material smoothly without forming bubbles. In a pinch, you can also use a paint pad; attach it to a roller extension handle so you don't have to work on your hands and knees.
Pour and Drag
Pour a line of oil- or water-based polyurethane along the wall farthest from the door. The wall should run parallel to the flooring. Drag the applicator through it along the entire length of the floor, from one wall to the other. Keep the applicator on the floor when you change directions to make another pass -- this prevents bubbles from forming. Continue pouring and dragging to cover the entire floor.
Let the Sealer Dry
Drying time depends on the product you're using. Water-based products take about two hours to dry, while oil-based ones can take from 10 to 24 hours.
Apply More Coats
Three coats is the standard recommendation for polyurethaned floors. Some products require light sanding--or scuffing or "screening" with a buffing pad--between coats; others claim that no sanding is necessary. Sanding or scuffing typically helps smooth out the preceding coat and helps the new coat adhere. Follow the manufacturer's directions.