Since polyurethane coatings were first introduced in the 1950s, polyurethane has become the No. 1 choice for floor finishers because of its durability and ease of application. Water-based polyurethane, a more recent development than the solvent-based variety, dries more quickly, thus making single-day floor finishing feasible. Although it takes longer for a solvent-based product to dry, both water- and solvent-based products take several weeks to completely harden -- or cure.
Oil- and Water-Based Drying Times
Often known as oil-based polyurethane, or OBP, solvent-based polyurethane is a type of varnish that contains resins of polyurethane, a synthetic plastic, instead of the plant alkyds found in traditional varnish. In water-based polyurethane -- WBP -- the resins are suspended in a water-soluble emulsion instead of directly in a solvent. WBP dries in a two-step process, the water evaporating before the emulsion containing the resin, but the process is much faster than that by which OBP dries. A floor coated with WBP is generally tack-free after four to six hours, while the same floor coated with OBP takes from 24 to 48 hours to dry.
Drying Vs. Curing
Solvent evaporation is only the first stage in the drying of a polyurethane finish. After the solvent is gone and the finish is hard enough to walk on, the urethane molecules begin to combine with oxygen and cross-link, and it's this process that creates the hardness for which a polyurethane finish is known. The curing process takes about the same time for OBP and WBP -- about four weeks. When it's complete, the coating is chemically inert and has achieved its maximum resistance to impacts from shoes and furniture as well as liquid spills.
Visually Assessing Dryness
An examination of the floor is usually a better gauge of dryness of the finish than referring to a clock, because drying time depends on environmental factors that include the temperature and humidity in the room. The best indication that WBP has dried is the absence of any coloration -- the milky white color of the liquid finish disappears when the product dries. OBP products aren't milky in liquid form; they generally develop a matte or satiny sheen when they dry, and you can walk on the floor when all the glossy areas disappear. Re-coating the floor while signs of wet finish are evident is a recipe for disaster -- the finish can bubble and separate, and you may have to strip it.
Timeline for Using the Floor
Fresh polyurethane can be walked on as soon as all signs of wet finish disappear. You may be able to walk on the floor as soon as two hours after applying WBP and eight hours after applying WBP. You should wait two days before moving furniture onto a floor finished with WBP -- four days for OBP. The curing process usually takes about four weeks to complete for both types of finish, although it can be less if the temperature is high and the humidity low. It's best to wait a full 30 days before laying down any area rugs, but if you're in a hurry, you probably won't have problems if only wait for two weeks.