Fur is a technique used to level walls and ceilings through fur channels. Hat-shaped channel steel is a kind of hat-shaped aluminum channel steel used to provide corrosion-resistant frame members for walls, masonry walls or ceilings. Masons and builders often use hat-shaped tunnels to level uneven walls or ceilings. Another benefit of aluminum channels is that they can protect your internal and external frame applications from fire, moisture, and corrosion.