Go here.. Chronic Progressive Lymphedema
And read what I have written. Your horse doesn't probably have CPL yet, but it is something you should know about. Treat for scratches first. Almost every horse we have brought in from England, had mites, so treat for that first. Keep the area clean and shave if necessary. You don't need to shave the whole feathered area, but just the area behind the pastern. Do you see your horse itching his legs on things or biting at his feet? If so, it is probably mites. Horses with problems within the feather, will also often stamp the foot a lot.
And don't feel bad. Our farrier does a ton of Shires and Clydes. He told us that every horse he trims, has scratches or CPL. Doesn't seem to have much to do with being wet either, since we are in S. California and there is no pasture. All are in dry sand lots usually.