Flickr/Joe Shlabotnik A sunscreen's sun protection factor (SPF) is only fully effective for two hours after you put it on. Experts recommend carrying a bottle of SPF 30 to SPF 50 sunscreen around with you, even on cloudy or rainy summer days, so you can throw some on if the sun comes out. You can still get a tan with sunscreen on, because while SPF blocks damaging UVB rays, it doesn't prevent deep-seeping UVA rays from penetrating your skin and giving it color.