Why does black plastic trim turn white after polishing?

Polishing compounds and waxes can dry inside the textured surface of unpainted plastic and leave a chalky white residue. In other cases, UV oxidation has faded the plastic itself. The correct treatment depends on whether the whiteness is product residue or material degradation. Residue usually sits in patches or lines that match where the polisher touched the trim, while oxidation fades the whole part evenly. Taping off trim before polishing prevents most residue problems.