Explainer states: "By way of contrast, one of the first manufactured sunscreens, Gletscher Crème, had an SPF of 2. This means that a melanin-challenged person would need a fresh rubdown every 20 minutes."

From what I understand, you can't stay in the sun indefinitely with SPF 2 simply by reapplying every 20 minutes. Instead, with SPF 2, you simply burn after 20 minutes -- reapplication or not. I don't think a "fresh coat" of a particular SPF gives you an extra allotment of time that that SPF protects you for.

In other words, if an SPF of 2 lets 50% of the UV-B in, how would you get extra time out of a fresh coat?

I thought the exact same thing. I'm quite sure you're right. Anybody care to give it a test this weekend?

Yes, you guys are right. Reapplication is not the issue -- the SPF number itself assumes that the lotion remains applied for the entire duration of the exposure. The reason for reapplication is that water and friction remove the lotion over time, making it impossible to achieve the labeled SPF unless you do reapply.