As a brand ambassador for Sawyer Products I was able to utilize several topical chemicals on the AT that I found essential to my health and success.
First of all, I used Permethrin to treat all of my clothing, gear, and shoes. This included the 5 pairs of shoes I sent myself in resupply boxes (and the many pairs of new socks I also mailed myself). As a result, I only found two ticks on me. They were both on my left arm at the same time, immediately following a thrashing through head high grasses near Pearisburg, VA. I flicked them off, no harm done.
Secondly, I used Picaridin insect repellent instead of DEET. As I said, no tick problems. The time I did find ticks I wasn't actually wearing Picaridin. The only times I felt like the Picaridin wasn't working was when I was sweating profusely in the northern Mid-Atlantic states. I'd apply the repellent and the mosquitoes and black flies would leave me alone for approximately 3 minutes. However, I'm not sure even 100% DEET would be sweat proof in those conditions.
I also used the Sawyer SPF 30 sunscreen in the northern part of the AT. I also use it when climbing and mountaineering. It doesn't need constant reapplication even in sweaty conditions, which I appreciate, and provides adequate protection from the sun.
For anyone considering a thru-hike of the AT I HIGHLY recommend treating your gear and clothing with Permethrin and using either Picaridin or Deet while on the trail. Ticks carry a lot of yucky diseases (not just Lyme).