Last time buddy got it on him mom used the orange listerine and it cleared up rather quickly, but he acted like it was painful(i'm guessing the alcohol burns like heck) i'd try something maybe a little more comfortable for the horse....
My horse has had two small bouts of rainrot in the time that I have owned him. What I did which worked extremely well was to bleach all my brushes after every time I brushed him. To treat the actual rot, I used a small diluted amount of betadine (please anyone, if this is not safe, please let me know), antibacterial/antimicrobial shampoo mixed with lot of water and spread it over the scabs, scrubbing with a stiff brush. The scrubbing loosened the scabs and really killed a lot of the bacteria. Then, while the scabs are still wet and soft, I gently pulled them off of my horse. Lastly, I applied a small amount of MTG directly to the affected areas. This protocal usually only had to be repeated for a few days, and everything was all cleared up. Hope this helps.
Its rain rot, and it is contagious.
So if you have it, make sure to use different saddle pads on your different horses.
And be sure to wash them after you use them on your infected horse.
I use bleach when May gets it. Washing before bleaching also helps!
At the University I am at we use an iodine bath with warm water, and a good curry comb. While the scabs are all wet it is easy to use the brush to take them off. The iodine cleared it up with one bath on the horse we bathed.
You could also use hot water on a sponge and a curry comb to get it off. The horses normally wiggle some because it doesn't feel great when you pull them off, but they will feel much better once it is gone.
Go buy a can of aerosol Tinactin. Spray it on once a day. It will clear it up. The Tinactin is an antifungul and when you use the spray it helps dry it. Do NOT use the ointment. WalMart carries it in their Equate line which is cheaper. I had one mare who would get rain rot every spring and fall and this is what I used. She was the only one out of 5 that ever got it.
I know it's a little more expensive, but you don't need to bathe them when using this when it's cold.