What a weird coincidence. Today, in my Cactuseaters inbox, I saw a message from someone out in Pennsylvania, asking about the wisdom of hiking with an umbrella to block out the sun and avoid "sliming excessive sunscreen all over the body." The person was especially interested in the idea of taking an umbrella on long hikes through hot and arid places.
How strange; I was just thinking about this very issue. Give me a couple of weeks and I'll post some thoughts when I return.