I"m pretty sure what you are experiencing is a buildup of materials created from the process of burning wood and cooking food that adheres to the grill surfaces. I've had this on my gasser, Weber kettles, WSM, and Akorn. Try crumpling up some aluminum foil and cleaning it off. It's pretty much a natural effect.
WSM, Green stainless Performer, Genesis 3000, 2 18.5" Weber kettles,
Hunter's safety orange Thermapen.