Hi, I can understand your concern for the problem of tan mark on hands. This is due to activity of melanocytes which causes deep pigmentation on aforesaid areas. Melanocytes are the color producing cells of skin. This is genetically determined in majority of cases and varies form person to person accordingly. The most important thing is sun protection by all measures. Wear full sleeves clothes for sun-protection. Eat healthy diet containing lot of Vitamin A and Vitamin C. Do not try applying over the counter products as these can further aggravate the situation. Protect yourself form direct sunlight. Use a good sunscreen on exposed area for 2-3 times and drink plenty of liquids everyday. The most important part in treatment is by lightening agents such as Kojic acid or azelaic acid cream application which you can use under dermatological guidance. All the best.