Bleach and ammonia degrade urethanes faster than any single factor. Never mix them in a refinished tub.
Suction-cup bath mats trap moisture and can pull up an uncured finish. Wait 30 days minimum after refinishing.
Abrasive scrubbers create micro-scratches that dull the finish. Non-abrasive only.
Sharp objects — razors, tools, dropped glass — cause chips that will grow if untreated.
Hot metal (curling irons, straighteners) can burn spots into a coating.