As mentioned above, rain rot is bacterial.
My 2 day cure is to massage MTG into the crusts and leave on for several hours or overnight. Next day curry with a metal curry or shedding blade and the crusts will come right off. Shampoo with chlorhexidene. My favorite at the moment is the CVS brand wound wash. It’s clear and suds up and gentle enough to be left on without rinsing.
All 3 of mine suddenly presented with hives then rain rot 2 weeks ago. I suspect we had an outbreak of midges. The fly population exploded that week, too. I messed about with Eqyss and Listerene for a few days then went back to that nasty-smelling old faithful, MTG.