I have a Percheron cross with a double coat like a husker. And he gets absolutely filthy. I use pure argan oil on his mane,tail and gnarly coat areas. It’s not greasy and absorbs quickly so it won’t hold in dirt. Most detangling products and coat sheen products contain silicone products and derivatives that actually dry out hair and skin creating an entirely new problem. Petroleum products like mineral oils, Vaseline and other stuff hold onto dirt and don’t solve the problem but make it worse.
Vitamin Shoppe, Amazon and health food stores carry pure argan oil. Put a few drops on your hands and work your hands in the wrong direction of his coat and then brush out. Heavily encrusted spots may take a few drops directly and time to penetrate.