How to tell if polyurethane has gone bad?

You can tell if polyurethane has gone bad by several visual, smell, and texture signs, as both oil-based and water-based polyurethanes have a shelf life of about 2 to 3 years unopened and about 1 year after opening. First, check for separation and skin, if there is a thick rubbery skin on top that does not dissolve when stirred, or if the liquid has separated into clumps and watery liquid that will not mix after thorough stirring, it has gone bad. Second, look for lumps, stringiness, or jelly-like consistency, good polyurethane should be smooth and pourable, if it is lumpy, stringy like snot, or has cottage cheese texture, it is spoiled. Third, smell it, if oil-based polyurethane smells extremely sour, rancid, or much stronger than normal, or water-based smells rotten, it may have gone bad. Fourth, test drying, brush a small amount on glass or scrap wood, if it does not dry within the normal time, stays tacky for days, or does not harden, it is bad. Fifth, check for mold or discoloration in water-based polyurethane. Always store polyurethane in a cool dry place with lid tightly sealed, avoid freezing for water-based. If in doubt, do a test patch before using on your project.