Whatever you do, don't do what my mom did.
When we bought my Ginger (and her mom) the two of them got the worst case of rain rot we had ever seen-- my mom had had horses all her life and had never seen anything like it. We were told to use hot water (not boiling) to soften the scabs and then pick them off. Mom went at them with a curry comb. I could see the pain in our horses' faces and they winced every time. Ginger's mom, Dixie, handled it well, but for months and months after that Ginger wouldn't let me brush her back end, where the rain rot had been the worst. Even now, over two years later, I still get a little nervous brushing her because I know she remembers how much that hurt.
She hasn't had any since, but man was is a pain in the butt. Good luck! And I'd recommend any sort of topical treatment these brilliant people tell you-- I wish I'd had this forum to come to earlier!!