That means that if you put on a second layer of the same sunscreen, it should reduce that 1/20th by another 1/20th, for 1/400th total reduction. If that were true, 2 layers of SPF 20 sunscreen would be equivalent to SPF 400 sunscreen! Five layers of SPF 20 sunscreen would be equivalent to SPF 3.2 million, enough to block the UV at the Sun’s surface.
