What will polyurethane not stick to?

Polyurethane will not stick well to many surfaces that are contaminated, non-porous, or have low surface energy, so proper prep is critical. It will not stick to surfaces contaminated with wax, silicone, grease, or oil, including furniture polished with silicone sprays like Pledge or wood that has not been cleaned, as these cause fisheye and peeling. It will not stick well to bare metal without primer, as it needs etching primer for adhesion. It will not stick to glass, glazed ceramic, and highly polished stone without special bonding primers because these are too smooth and non-porous. It will not stick to plastics with low surface energy like polyethylene, polypropylene, Teflon, and some vinyls without special adhesion promoters. It will not stick to previously finished surfaces that have not been sanded or deglossed, as the new coat needs a scuffed surface to grip. It will not stick to damp or wet wood, as moisture prevents curing. It will not stick to surfaces with peeling or flaking old finish that has not been removed. It will not stick well to raw silicone caulk or to surfaces with heavy dust. For best adhesion, always clean thoroughly, sand with 120 to 220 grit, remove all dust, and ensure surface is dry and free of contaminants.