Tends to be called Rain Scald in the UK. I use Hibiscrub which is chlorhexidine in a soapy base. It's used in hospitals for hand cleansing and skin prep but is brilliant for rain scald. You would probably be able to buy it at your local pharmacy. Just wash the affected area with a handful of the liquid, remove the scabs (they usually soften pretty well with this stuff) and rinse with clean water. Treat on a daily basis until it is clean and dry. If that doesn't control it the next step up is Betadine surgical scrub (povidone iodine), use it the same way and agian it will be available from your pharmacist.