Cleaning metal parts?

What is everyone using to clean and polish and maintain fuel and radiator cap and other parts like this?

FWIW, I cleaned the brushed aluminium bits to a nice state with some 3M steel wool substitute then clear coated them in a lacquer. Just made sure I carried on in the direction of the existing buffing marks, else it would look crap.
Stainless Steel soap pads also work great on aluminium leaving a brushed look, but it's way more aggressive, so you have to be more careful using these, but they get the job done quick smart.

I applied a ceramic coat to the aluminum bits when I coated the rocket. Didn't think of lacquer - will have to give it some thought

I just use a good non-abrasive (or very mildly abrasive) metal polish, on a cloth, not wire wool.
I bought my bike online sight unseen from a dealer. I was quite disappointed when it arrived because the brushed aluminium parts were tarnished. Not actually furred but blackened and dirty looking and it wouldn't come off with washing.

The best metal polish in the UK is reckoned to be Meguiars All Metal but I couldn't get hold of any on the high street so I bought a bottle of Turtlewax All Metal instead. It worked really well.

All these brushed metal parts are lacqured. Unless they're furred and severely corroded I wouldn't be tempted to rub through that lacquer with agressive abrasives because then you're committed to polishing and re-lacquering, otherwise they'll deteriorate rapidly.
A lot of these marks will be surface. I'd try a gentle metal polish first before reaching for wire wool etc.

lemon and salt