1/3 dish washing detergent (because it rinses out so easily and leaves no residue)
1/3 mouth wash (antiseptic)
1/3 white vinegar (low PH and antibacterial/acetic acid helps dissolve scabbing.)
When you wash the horse, scrub affected areas, let soak for about 5-10 minutes and rinse off. The horses coat shines and is soft when it dries. I've seen it help REALLY bad rain rot.