Again many thanks to my old fashioned yard owner. If those scabs are hard and stuck, rub in a small quantity of cooking oil and leave for 12 hours or so. Repeat as required and they will soften beautifully and without stress to horse. Once softened they can be lifted by gentle scratching and will gradually fall off, with the clumped hair, as the new skin underneath develops. Horse finishes up unstressed, your hands are soft and rainscald healed. It works under rugs but I would strongly advise putting an old sheet or something under the rug and fastened with a surcingle as the oil makes a right mess of the rug lining. Once healed send grotty rug to be cleaned and disinfected and use a fresh one.
Connie loves having her bum scratched but will not tolerate a hibiscrub treatment. She told me what to do with the idea in her usual unmistakeable manner.