Being a nudist, you wouldn't expect any tan lines on my bod, which I get tanned all over at our clothing optional Haulover Beach. But after biking around in thongs all summer, seems I have got tan lines after all. (they would be even more pronouced - except my time tanning at the beach is totally barefoot.)
Minimize tan lines by wearing a different type of flip flop or sandal that exposes the areas that tend to be covered!