I use betadine, I make sure it really gets thoroughly wet. I do this once a day for two days. Then I use a mixture of 1/3 listerine (original, not mint or flavored,) 1/3 apple cider vinegar, and 1/3 water. I spray it on and again make sure it is really wet. One time a day for three days. I was dubious about this, but someone just swore by it, so... why not try it. Rain rot is gone, hair is almost grown back in, total time since start of treatment is 9 days.
Now, my environment is a dry desert. When I got Christy, she had rain rot, it went away on it's own once I kept her legs dry and clean. But... this spring it came back and really spread on her legs. We have not had hardly any rain... ? Anyway, she no longer has rain rot. Just though I post this... In case anyone want's to try it. It's pretty cheap.