Makeup artists everywhere stress the importance of priming. Makeup primers are crucial at keeping oil at bay while filling in fine lines and providing a smooth canvas for your foundation to hold to. Once you've got the right formula, apply the primer after all your moisturizing. "It's like waterproofing your skin." If you apply it before your day cream or sunscreen, those water-based products can't penetrate through to reach your skin. If you smooth the primer on after, your skin can absorb your moisturizer before the primer forms a barrier on top of it.