Wash thoroughly with hibisrcub on the affected areas, leave the hibiscrub on for a few minutes to soften the scabs, then wash off and try to gently pull off as many scabs as possible. Then dry thoroughly (use a towell as this will help loosen a few more scabs), if the area isn't raw then I'd probably leave it open but if it looks infected and sore then you can apply an antibacterial cream like fungatrol or similar. If it's only minor then will probably need only one treatment but obviously keep checking horse to ensure it doesn't reoccur.
You should always dilute hibiscrub. Only time I have seen it used neat was on the vet's hands prior to a gelding op.
The other way to remove the rainscald is cover the area with any cream such as sudafed, wrap clingfilm round then a bandage and leave 24 hours.
When you remove the covering rub gently with an old towel and all the scabs fall off with the towel.
repeat until the legs are clear of scabs, then smooth nappy cream in to make them heal.