I've found deworming with Panacur's Power Pack helps horses blow their coats. My TB was going through a normal shedding this year and I dewormed him. About a week and half to two weeks, his remaining coat was shedded out. Last year he took forever to shed out too.
We spray our horses down daily and it seems to help a lot however this spring was our first spring on an all forage diet and our horses completely dropped their winter coats in two weeks. They went from shaggy wooly mammoths to complete summer coats in no time. I was shocked because we have one TWH who never completely sheds out till about july and he is just as slick and black as can be.