How to Apply Polyurethane for Floors

Floor Finishing

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Overview
  • Working Time: 6 - 8 hrs
  • Total Time: 6 days - 1 wk
  • Yield: 500 square feet
  • Skill Level: Beginner
  • Estimated Cost: $120 to $200

Your beautiful solid hardwood or engineered wood floor deserves the best possible finish. Polyurethane floor finishes are easy to apply, and they cure rapidly. They offer rock-hard, waterproof, long-lasting protection for flooring, plus they let the beauty of the natural wood show through.

What Polyurethane Finish Is

Polyurethane finish is a liquid resin used to coat wood flooring and other wood surfaces, such as cabinets and furniture. As a synthetic floor finish, polyurethane is made from raw materials that are derived from crude oil. 

Polyurethane floor finish comes in either water-based or oil-based versions. Water-based polyurethane is easier to work with and quicker to dry, but wears down faster than the oil-based finish. Oil-based polyurethane finish, while hard and durable, is noxious when applied and takes at least twice as long to dry.

Which to Buy?

Oil-based polyurethane finish is more work during and directly after application, but this is balanced by the need for fewer reapplications and less maintenance. Meanwhile, water-based polyurethane is less work at the beginning, but it does require frequent touchups and reapplication about every two years. The frequency of reapplication depends on the amount of traffic on the floor, along with any potential damage from pets and UV rays.

Polyurethane finish is transparent. It comes in a range of sheens, from flat to glossy. It resists fungus, mold, and mildew well. One feature that makes it particularly good for flooring is that it is waterproof. Because polyurethane dries to a hard finish, it's especially good at resisting scratches and scuffs.

Basics of Polyurethane Floor Finishes

Oil-Based vs. Water-Based Finishes

Oil-Based
  • Slow drying time

  • Cleans with mineral spirits or paint thinner

  • Extremely hard surface

  • Pungent odor

  • 500-600 square feet per gallon

  • Best for all areas, including high-impact areas

  • Transparent, but with a slight amber tint

Water-Based
  • Fast drying time

  • Cleans with waer

  • Hard surface, but not as hard as with oil-based finishes

  • Low odor

  • 260-320 square feet coverage per gallon

  • Best for moderate or light-use areas

  • Completely transparent

Cost of Finishes

Both oil-based and water-based polyurethane finishes cost about $50 to $70 per gallon.

To apply two coats of oil-based finish to 1,500 square feet of wood flooring would cost about $300 to $420 for the finish alone. Water-based polyurethane has a lower coverage rate due to evaporation, so it would cost about $450 to $550 for the finish.

Number of Coats

Most manufacturers of polyurethane finishes recommend a minimum of two coats to achieve the desired level of durability and sheen.

Drying Times

Water-based polyurethane floor finishes dry to the touch in about 2 hours and can be recoated in about 4 hours. For oil-based finishes, double the drying time. For either finish, the floor should be safe to walk on after about 48 hours. Allow one week for the surface to fully cure.

Temperature, humidity, and ventilation affect drying times. Keep the room between 55°F and 90°F and maintain a relative humidity of around 50 percent. If necessary, increase humidity with a humidifier or decrease it with a dehumidifier. Ventilating the room with a fan on a low setting will expedite drying.

Alternatives to Polyurethane

Though polyurethane is the most popular type of on-site floor finish, some alternatives offer other advantages and disadvantages:

  • Penetrating Oil: Penetrating oil soaks into the wood and requires several coats. It sheds water but isn't good at resisting scratches.
  • Varnish: Made from wood sap and alcohol, varnish predates polyurethane as a floor finish. Varnish has more solids than polyurethane, producing a thicker coating.
  • Wax: Wax is spread in multiple thin coats after the stains have been applied. Then, it is buffed to the desired level of gloss. Though wax is inexpensive, it does require frequent reapplication.
  • Aluminum Oxide (Pre-Finish): Aluminum oxide is an extremely hard finish that rivals even oil-based polyurethane's durability. The downside is that it cannot be reapplied on-site; it can only be applied in the factory when the floor is being manufactured.

Safety Considerations

Sanding, scraping, or removing old paint on floors can be hazardous if that paint contains lead. Lead is toxic and can cause illness and even brain damage in children. Always wear a NIOSH-approved respirator. Clean up with a HEPA vacuum and a damp mop. Adhesives may contain asbestos. If you believe that the surface may contain asbestos, have it tested before sanding it.

What You'll Need

Equipment / Tools

  • Floor sander
  • Floor finish applicator with pole
  • Clean old rags
  • Breathing and eye protection
  • Broom
  • Shoe covers or old pair of socks
  • Tape measure
  • Paint brush
  • Painter's tape
  • Paint tray and liners

Materials

  • 2 gallons polyurethane floor finish
  • Mineral spirits

Instructions

  1. Plan Application

    Calculate the area of the space by measuring the length of the room and the width of the room, then multiplying the two numbers. Subtract any large obstructions, such as fireplaces. Add 15 percent to the square footage for wastage. Purchase the polyurethane finish based on this number, making sure to account for multiple coatings.

  2. Move Furniture

    Remove furniture and all other objects on the floor such as rugs. Move these items into another room. Broom-clean the wood floor to remove large debris.

  3. Sand Floor

    Use an oscillating floor sander or a drum floor sander to remove the existing finish if it is in poor condition. Avoid using steel wool, as this may cause rust to develop under the finish. Floors in especially poor condition should be sanded with the drum sander with sandpaper in the #100 to #150 grit range, working up to a #220 grit paper.

    Warning

    If your floor is an engineered wood floor, its top veneer surface is extremely thin and likely cannot be sanded with a drum sander or with any coarse grit paper below #220 grit. If the floor has been sanded once or twice before, you may not be able to sand it again.

  4. Clean Dust

    Use a HEPA-grade vacuum to remove all of the dust from sanding. Wearing latex gloves, dampen old rags with mineral spirits and use them, by hand, to remove the rest of the dust. From this point forward, keep your feet clean when walking on the sanded floor by using shoe covers or socks.

  5. Tape Walls and Trim

    Apply painter's tape to walls, trim, and other large items that cannot be removed. Apply the tape tightly to prevent the polyurethane from seeping below the tape.

  6. Cut in Edges and Corners

    Stir the polyurethane gently. Do not shake. Dip the brush in the finish and apply the finish to the edges and corners, up to the painter's tape.

  7. Finish Floor With Applicator

    Pour the polyurethane finish into a lined paint tray. Fit the applicator pad on the applicator. Dip the applicator in the polyurethane. Brush the polyurethane onto the floor in a W-pattern until complete. Make sure that you are using only an applicator intended for floor finishes. Cotton and other types of applicators may leave lint behind, which can become sealed under the finish.

  8. Sand Before Recoating

    Let the finish dry until it is dry to the touch and hard. If the previous coat is more than three days old, lightly sand the floor with #220 grit sandpaper and thoroughly clean.

  9. Recoat Finish

    Recoat the flooring, repeating the previous steps. Be sure to cut in the finish before applying the finish to the main section of the floor.