Polyurethane is a highly durable polymer commonly used as a form of insulation. Polyurethane can also be harnessed as a foam and a liquid, and it makes up most wood floor finishes. If you want to clean a stain that appears on a polyurethane-coated surface without ruining the finish, you must use caution. Harsh chemicals and acids can dissolve polyurethane, leaving you with an unfinished or inadequately finished wood surface.
Mix a cup of water with a drop of mild liquid detergent. Mild detergent contains plenty of cleaning power, in the form of acids like phosphoric acid, but is not corrosive enough to damage a polyurethane finish on a wood surface — as long as you use it in moderation. If treating a stain on a wood floor, dip a sponge in your mixture and wring it out to remove most of the moisture. Wood floors can warp if you saturate them.
Blot the stain gently on your polyurethane surface. Scrub gently if necessary, but do so only using a soft sponge. Scouring pads and abrasive sponges can scratch wood surfaces and ruin urethane finishes. If your stain lifts, rinse away the residue with a damp sponge. Otherwise, proceed to the next step.
Dampen a sponge with a polyurethane floor cleaner. These cleaning solutions, sold in home supply stores, are specially formulated to preserve polyurethane finishes while removing stains. Scrub the stain gently with your soft sponge, dampened in the solution, and rinse the surface with another damp sponge. (Remember: Never soak your sponges or clean a wood floor using a dripping sponge.)
Dry your polyurethane surface by hand, especially for wood surfaces. Wood can develop hard mineral deposits if not properly dried, and these stains can sometimes show up more prominently than the stains you previously removed. Wipe away the liquid from the surface with a soft towel. Any remaining dampness can dry on its own as long as you have ample air circulation, so open windows or turn on fans until the finish dries.