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You have got a sever sun tan because of long hours of sun exposure.The best thing is to wear full sleeves along with a good spf sunscreen daily. You should use the sunscreen every three hours in the day.This would protect you from further damage.
For treatment ,you can go for chemical peels
,along with topical application of hydroquinone
cream at night.You can consult a dermatologist
who is experienced in doing chemical peels .