How can I refurb aluminium iPhone back covers?

Hello everyone,

I have been fixing smartphones since 3 years and i have kept the old back covers of the iphone (5, 6 and 7 series) since then. I now have hundreds of them which are partially or quite scratched.

I would like to know if it is possible to refurb them to make them look new or at least cleaner?

These are originals so it is clearly better to use them than the other I can buy.

I have tried toothpath but it only works on little scratches.

Does anyone have a real method?

Please, it would help me a lot (and a lot of people i believe)

Thanks,

Aimé