My Hypalon RIB had a bad case of mildew and grunge that would not be removed with the strongest detergents commonly recommended for dinghies. It also chalks after a month or two between washings. I was going to paint it this year with Tuff Coat. The TC directions said mildew must be removed prior to painting and recommends a 1:3 solution of bleach to water.

I would never have tried bleach because it's so caustic but I was giving up on this -- it's a miracle! The dinghy now looks so good (almost like new other than wear spots) I decided not to paint it and it's been launched as-is. Perhaps I'll paint it next year or before I put it away in Fall.