It all adds up!
Have you ever gone out for the whole day on a boat or an outdoor trip then come home and nursed your sunburn saying, “I don’t know how that happened I was wearing SPF 20 all day!”
It’s like a microwave cooker. When you put something in that takes 20 minutes to cook, it doesn’t come out after 18 minutes still stone cold and suddenly cook when the clock hits 20 minutes.
When you put on SPF 20 this gives you 200 minutes until you start to burn, so 100 minutes later you are halfway there. Even if you re-apply the sunscreen your skin has already taken some punishment and won’t go back to square one today.
If you know you are going out in the sun for the whole day and you don’t want to get burned, start with a very high factor sunscreen and after a while you have no choice – cover up!