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Once upon a time, back when I was more actively painting, I knew the answer to this question but I've since forgotten. You know how with the metal minis, especially the older lead before the pewter mix came in, if exposed to open air they eventually become a little tarnished, be it yellowish or gray or just dirty looking. What is the easiest way to clean them up so they are in best/cleanest condition for priming/painting again?

any kind of metal polish should work nicely

I believe (and if someone has better info, please chime in) that metal polish leaves a protective film behind that might affect paint and glue adhesion. Maybe a quick rinse with denatured alcohol would strip this barrier.
I've heard that a paste made from flour, salt and vinegar is also good at removing tarnish from metal. Apply, let set for an hour or so and rinse.
I've also read that allowing the piece to soak in Coca Cola for a couple of days will do the same. Don't know about Pepsi or off brand. Should work, I think.
Let us know how any of these work out for you.

Give toothpaste a try! Use an old toothbrush and give the mini a scrub with some toothpaste. It has worked for me in the past for miniatures and is my go to polishing technique for silver.

Art!

I've had minis forever, that includes the old GW and D&D minis. the old leads didn't tarnish. It's whatever else is in them. The 'newer' old GW were the worst. They were the 'white' metal, which probably mean more pewter. I know pewter itself tarnishes, they sell cleaner for it.

I've just cleaned them up with soap and water, then sprayed them with a metal primer, either white or black; sometimes tan.