today was a hot day- low 80s from noon on, (it's 83 now)
I wore my arm coolers today;

I was out in the sun from 11am until 5pm with those things on. The first hour was cycling, 1 hour of sitting around, and 3 hours of walking up and down hills on a street scramble. (Ah the hills of Seattle!)

I decided that by 5 that the sun would be cool enough to take them off - so I did. To my surprise, the sun felt so much hotter on my skin that I put them back on. They do really work!
I didn't have to put sunscreen on all day and I am not burnt. Of course it's only SPF 30 so if you're sensitive, you might still need sunscreen.