To really do a 'number', I'd use 3m Finesse-IT compound. You'll need to apply it with a machine polisher with ideally a 'super-mop' head. It'll make a godawful mess and will need to wash and hose the whole issue off after. Then, when dry, apply a decent GRP polish. Unless the fading/oxidisation is particularly bad, this process should bring it up like new.