Polyurethane varnishes are synthetic, plastic-based coatings that adhere to the surface of a wood floor, providing a tough, water-resistant surface. Wood floors finished with polyurethane require less maintenance than floors finished with wax or oil, and are more durable than floors finished with shellac. Polyurethane dries more quickly than natural oils, but more slowly than shellac or lacquer. The specific drying time for polyurethane on a floor depends on environmental conditions and how the finish is applied.
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Applying several coats of polyurethane to a wood floor provides a more durable, long-lasting finish. However, these coats shouldn't be allowed to dry fully. DuraSeal recommends allowing each coat to dry for at least eight hours and for no longer than 24 hours. This leaves the polyurethane slightly uncured, allowing it to bond to the next layer. If a coat does dry completely, screen it thoroughly before applying the next layer. This creates a rough surface that successive coats of polyurethane can bond to and reduces the risk of flaking and peeling later.
Even though a polyurethane floor may seem dry to the touch, this doesn't mean it's ready to walk on. Polyurethane finishes, like other types of plastic, must cure before they're completely hard. Allow a floor to dry overnight for light foot traffic, and for at least 48 hours before replacing furniture or allowing heavy traffic. Failure to let the polyurethane finish cure correctly could result in smudges, dents in the finish or later peeling.
Not all polyurethane floors dry at the same speed. If the environment is humid, cold or poorly ventilated, a given finish could take longer to dry. Higher temperatures, dry air and good airflow can reduce drying times. Heating coatings can make them dry faster, but this increases the risk of fire from airborne vapours. Maintain good airflow and use proper safety precautions when drying polyurethane on a hot day or using supplemental heat.
Thinning polyurethane with naphtha or other fast-drying solvents may appear to shorten drying time. However, no chemical additive can speed up the second phase of polyurethane drying: the curing process. This process requires chemical crosslinking through contact with oxygen. A floor may look dry without being properly cured. Applying a thinner layer can speed drying time, since oxygen permeates the coat more quickly, but most floors will require additional coats if finished with thinned polyurethane.
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- DuraSeal: DuraSeal Polyurethane
- Paints & Coatings Resource Center; Curing Time of Polyurethane Coatings; Ron Joseph; October 2002
- "Popular Woodworking Magazine"; The 7 Myths of Polyurethane; Bob Flexner; June 2, 2008
- The Flooring Lady: Finishing Hardwood Floors
- Massachusetts Coalition for Occupational Safety and Health; Protecting Workers and Homeowners from Wood Floor-Finishing Hazards in Massachusetts; September 2005