the bacteria causing rain scald lives under the scabs and forms laminated layers of hyperkeratinised skin - which cause the horrible crusty scabs of rain scald and mud fever. It's important to get the scabs off, however they will be sore and unless sedated I very much doubt your horse will put up with having them clipped off!! The hair is on top of the scabs and as such will come off when the scab does, which is fine as it will grow back. Keep your horse well rugged, dry and out of the rain, and try an antibacterial scrub like hibiscrub to disinfect the area - it must be left in contact with the area for at least 5 mins to take effect. Creams such as aromaheel can be very effective in both removing the scabs and soothing the area.