Does anyone know how to prevent the soles from hardening and getting slippery? Or reviving old soles? I just replaced 18-month-old shoes that had perfectly fine uppers, only because they lost their grip. At ~$90 that's not acceptable.

I once tried using the sanding belt and the fresh surface wasn't good either. It seems the soles age in 18-24 months and then the shoes are dangerous on a boat.