A product called MTG is great for getting rid of rain rot. Warm bath to soak the area, scrub with a curry comb and anti bacterial shampoo. Once dry apply MTG (it's an oil) let soak in. Apply daily and keep removing scabs.
As for dandruff, daily deep grooming and a coat conditioner. Added omega fatty acids to the feed to improve the horses coat. Such as soaked Flax seeds (whole or ground, ground is my bio available), flax seed oil, fish oil, cocasoya oil.
I find if you oil the lesions first (baby oil or coconut oil), it's easier to take off the scabs which can be necessary for healing. Then wash the lesions and the oil off with an antimicrobial shampoo. Make sure you scrape off excess water after and allow the horse to dry completely before repeating (after all, rain rot is a bacterial infection caused by spores that can penetrate the skin when it's wet). If it's very bad, you may need to get an injection or a stronger shampoo from your vet. Most of the time it clears up on it's own though.