Update :
On July 1 I tried a quick clean up using toothpaste and soft brush on the case. It can easily remove all the tarnishes and whiten the whole case back to almost as new.
Not really recommend this to everyone cause someone says the abrasive from toothpaste will ruin the case in the long run.
And now a week later, July 7. It seem the tarnishes still haven't come back yet.
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