When not to use polyurethane?

You should not use polyurethane in several situations where another finish would be better or where it can cause problems. Do not use polyurethane on cutting boards, butcher blocks, or wooden utensils that will be directly cut on, because the hard film can chip and end up in food and it is not designed for direct cutting, use food-grade mineral oil and beeswax instead. Do not use polyurethane outdoors on exterior furniture, decks, or exterior doors without UV protection, because regular polyurethane will yellow, crack, and peel from sun exposure, use spar urethane or marine varnish with UV blockers instead. Do not use polyurethane over waxed or oily wood without proper prep, as it will not adhere and will fisheye. Do not use oil-based polyurethane over light woods if you want to keep them light, as it adds an amber tone, use water-based polycrylic instead. Do not use polyurethane in very humid or cold conditions where it will not cure properly. Do not use it on floors that need to be food safe for direct contact or on surfaces that need to remain breathable. Do not use polyurethane if you want a natural, matte, hand-rubbed feel, as it creates a plastic-like film, use tung oil or hardwax oil instead.