I've tried numerous sunblocks (from spf 30 to 55, in sweatproof, sport, dry touch, waterproof, etc) and no matter what I wear and how often I apply, I still get pretty severe tan lines after 3+ hours of biking. I figure that I'm filtering out most of it, but short of not going out at all or wearing more clothing, I don't know how to block all the rays.

Sorry that I'm not much help! I'd be interested to hear what your dermatologist recommends/suggests.