There really isn't any fast way to get rid of rainrot, if it's chronic.
Changing the horse to a bland diet does help, while still making sure it's receiving the appropriate vitamins/minerals. Especially those vitamins for the immune system.
Now that I've said that, my Arab is prone to rainrot, no matter what I do.
I've changed his diet several times; things have improved but there is just something about his system that he is a sweet itch and rainrot magnet
I use MalAcetic shampoo. It's expensive but it doesn't take much. You have to order it on-line, unless you have a vet that carries it or is willing to get it for you. "DermaPet" MalAcetic Shampoo for Dogs and Cats (ONE GALLON)
It can also be bought in an 8 ounce bottle. I use it on the dogs and four horses, so I bought a gallon in 2010. I still have half of it left as I don't use it all the time or I will just use it, as needed.
I try and brush the Arab every day either with a stiff bristled brush or a hard plastic brush --- be sure to wash in soap & water after EVERY use.
As long as his skin isn't broke open, I keep a 50-50 mix of generic, original Listerine & water in spray bottle and spray the affected areas daily.
He only has issues in mid-to-late-summer when the heat/humidity are high and he's starting to shed for his winter coat. Yes, he's been shedding since mid-July; he's always the first one to start
We live in the Tennessee Valley, where allergies of any sort prevail and that doesn't help. Meaning, your environmental living conditions could be playing a big part in the rainrot.
You will get a lot of input, as what works for one horse doesn't always work for another. The constant for every horse is to brush them daily, if possible, and always wash the grooming tools in soap & water after each use. Don't share those grooming tools either
Hope this helps.