We dealt with scratches all summer last year with my arab gelding. This is what we did:
Clean and shave the affected area. Soften and remove the scabs. You can use a mild betadine wash, but beware, scratches can be extremely painful for the horses so you will probably need an extra hand. Once everything is completely dried (i towel dried my guy) you need to apply either swamp juice which you can get from your vet, or make your own (recipe below) to a gauze tefla pad. Depending on how large the effected area is, you may need a larger or multiple gauze pads to cover it. Then cover the area with a cotton banadage, and then vet wrap or elasticon. be sure you have cotton under the vet wrap, as it will continue to tighten and could cut off circulation. If its low on the horses leg, such as on the coronet, you can use duct tape to wrap the bottom and secure it. The horse can be turned out as normal. Replace the bandage and reapply the salve every 2 days for the first week, and then every 3 to 4 days there after until the scabs are gone.
The reason for bandaging is because the bacteria and fungi which causes the scratches is photo-sensitive, meaning it flourishes with the light, hence why you see it on white/light skin areas. Any moisture will also amplify the fungus/bacteria
We started our treatment with swamp juice from the vet. its very caustic! and green, hence its name. I don't recall whats in it. we did however make our own paste later that worked just as well, and if I remember correctly it was a mixture of equal parts triple antibiotic ointment (such as neosporin), hydro-cortisone cream, and desitin diaper rash cream. Apply enough to coat the cleaned scabs.
Hope this helps!