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Usually dead skin build-up plus dehydration. Both are fixable in a few weeks, and neither needs a brightening serum to solve.

Two or three times a week, at night. More than that on most skin causes irritation, which makes tone look worse.

Usually the opposite. Over-exfoliated, irritated skin looks blotchier than skin on a simple consistent routine.

If you can point at specific marks, go to Dark Spots & Pigmentation. If it's general dullness and patchiness with nothing specific to target, you're in the right place.