i had a go myself on a rocker cover. got over it, took rocker covers, and engine covers to a back yarder guy he polished them all for $150 (5 pieces) over a period of time it does dull but theres products you wipe on, wipe off and it comes up like new again.
if you get them chromed, they stay mirror clean all the time but its a bit more expensive but better mirror reflections.
mine: