My only iron race was in November, so the Sun was not a huge factor - It got dark earlier, and for the most part I was mostly covered up on the bike. That said, I seem to recall that I re-applied in transition. I once heard, but cannot confirm, that if you put on some sunscreen before you go to bed the night before, and then re-apply in the AM, that it will last a bit longer. I have no idea whether or not that is fact or fiction however.