I use MTG, and it works wonders, and doesn't wash off in the rain. I normally will take a shedding blade, and just very gently lift off what scabs I can, as you need to get whatever medicine you use down to the skin, so removing the scabs that come off easily if she'll let you remove them will help. I cover the entire area of scabbing with MTG on my mare, and leave it on, and within a week it was gone. She was one that wouldn't let me really mess with her rear end, so it worked great even with the scabs still in place. My Thoroughbred would get rain rot, and I'd just take a shedding blade, remove the scabs, and wash her with betadine or another antifungal/anti bacterial shampoo, and I don't remember what I put on it.