Caliper Piston Rusted

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I've never pulled apart a caliper before so I have a question - the inside of my caliper piston is showing some surface rust - see the picture

Is it OK, obviously some water has got in behind the pads, the rest of the caliper seems fine, clean, seals in good condition

Do I get a couple of rebuild kits or are they OK to leave like that?

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Thanks guys

The rust must have been from the bike's previous life in Canada, it's never going to see rain again LOL

I will get the sandpaper out at the next opportunity