Are you referring to the vinyl (or pvc) part of the rubrail or a SS insert in the rubrail "holder"? It sounds like the other guys are referring to the vinyl part and if you are, too, then another alternative is to use a mild rubbing compound (maybe even just a cleaner wax) which, like removing oxidation from gelcoat, will remove the same from the vinyl. Then wax it a few times a year, like you do the gelcoat, and it should stay looking nice an new for a long time. Don't go too fast or hard if you use a rotary buffer since it could burn it if you're not careful. But, a rotary will produce excellent results very quickly. I have not used acetone on something like that, but I suspect that a rubbing compound might give longer lasting results as it's completely removing the oxidized layer. But, that's just a guess.