How do I remove these "Stains" from my Krones?

[Blib]

I have a set of Eta Beta Krones fitted to my Porsche 964.

They've developed stains on the aluminium rims. I don't want to dive in with a product in case I scratch them.

What would you suggest to make them nice and shiny again? And how can I prevent it happening again?

Thanks.

[Blib]

Thank you for your help.

[Blib]

Well, I used the twins and my wheels have come up a treat!
.