Here's what works for me, when my bleached-white-by-Seattle winter legs hit their first REALLY sunny day: Elta MD 50 block. It's an all-zinc oxide (physical block) that prevents my legs from getting so much as a tan line along the edge of my shorts after a 3-hour ride. While I haven't tested it in the mountains yet, it is definitely the strongest sunblock I've ever used.

Drawbacks: it's heavy AND waterproof, and takes an insane amount of scrubbing to wash off at the end of the ride. And, despite the fancy nano-tech, I find that it doesn't completely soak in, and makes my legs look slightly paler. If you can deal with all of this, it will prevent sunburn.

Elta also makes a 45 and 30 block, and you might want to try one of these and work up. I've only found the stuff online or at a local dermatologist's office, so it's also a pain to find.

Good luck!