Coconut oil can be used to lighten the armpit. You can simply rub a small amount of coconut oil everyday on the darkened area. Make a mixture of rose water and pure sandalwood powder to form a paste and apply it in the underarms, leave for 20-30 minutes and wash it off with cold water.oconut oil can be used to lighten the armpit. You can simply rub a small amount of coconut oil everyday on the darkened area. Make a mixture of rose water and pure sandalwood powder to form a paste and apply it in the underarms, leave for 20-30 minutes and wash it off with cold water.