Why does faded trim turn gray again a few washes after using a dressing?

Most dressings sit on the surface of the plastic and darken it temporarily; washing and weather remove them, and the faded plastic underneath reappears. The more oxidized the plastic, the faster that happens. Clean the trim thoroughly before applying anything, because dressings do not bond well to dirty or chalky plastic, and consider a dedicated trim coating, which is designed to last considerably longer than a dressing.