Protects you from the sun – Getting overexposed to the sun can cause hyperpigmentation and sun spotting. A vitamin C serum will counteract any harmful effect of the sun and also decrease the chances of skin cancer by thickening the dermis layer. But it is important to note that a vitamin c serum cannot replace your sunscreen. A sunscreen provides the much needed SPF protection by shielding the outer layer of the skin while the vitamin c serum acts as a sun barrier from within the skin. You should apply the sunscreen as well as the vitamin C serum before going outside for prolonged periods of time.