The urethane used in sealing wood surfaces is more commonly known as polyurethane. Whether you can seal a prefinished floor with polyurethane depends on your circumstances and the condition of the hardwood floor. Before you attempt to apply polyurethane to a prefinished floor, contact the flooring manufacturer to gain a better understanding of how to work on your floor.
Although polyurethane is very commonly used, it is a dangerous chemical and should be handled with care. Polyurethane contains isocyanates, which are carcinogenic. Prolonged direct exposure to polyurethane and its fumes may cause long-term respiratory issues and, in some cases, lung cancer. When using polyurethane, work in a well-ventilated area and wear gloves, goggles and a face mask. If you are uncomfortable handling toxic chemicals, hire a professional to work on your floor.
Refinishing a Prefinished Floor
If your goal is to strip and refinish your prefinished floor, consult the floor manufacturer before proceeding. If the prefinished floor is solid wood, sand down the floor and then restain and reseal the floor with urethane -- there's no need to worry about damaging the floor. If your floor is made of engineered wood, you may still be able to strip and refinish it yourself, depending on how thick the top layer of the wood is. Most engineered hardwood floors can be refinished two or three times before they need to be replaced, but this depends on the floor. Consult with the manufacturer.
If the floor has only been stained and has not been sealed with a protective top coat, such as varnish or polyurethane, you can seal the floor with polyurethane once the floor has been installed. Polyurethane is best applied to the floor using a foam or lamb's wool applicator with an extension pole attached. These tools can be purchased at most hardware and home improvement stores. Spread the polyurethane evenly and thinly. Two coats is typically enough to seal most surfaces, but you may want to apply as many as five coats if the floor gets a lot of traffic.
Prestained and Sealed Floors
In general, you shouldn't apply polyurethane over a floor that has already been stained and sealed with other products. If the manufacturer indicates that the floor has been treated with polyurethane but you want to apply additional coats for added protection, you may do so after the floor has been installed. The floor should be wiped clean, lightly sanded with a very fine-grade sandpaper and wiped with a damp cloth. Once the floor has been scuffed, additional polyurethane may be applied as if the floor had only been stained.