Can Polyurethane-finished wood surfaces be damaged by water from cold glasses?
Polyurethane is more water-resistant than oils and other varnishes, but you can still get water damage if the water is left in contact with it long enough. Some marine polyurethanes are virtually waterproof, but nothing will withstand water indefinitely. Polyurethane is a good choice for a long-lasting, low maintenance finish – but you should still use a coaster. You will get the best results with it if you thin the varnish with turpentine or mineral spirits, and apply it in thin coats. Sand with a fine (400 grit) paper between coats, and make sure you have plenty of ventilation. Read the label on the container; you must usually apply polyurethanes at temperatures above 55 degrees F. I don’t recommend water-based polyurethanes for furniture, although they can be used on some other, low contact surfaces like wood shutters. Sources: Lots of woodworking projects.
