I have had this grill for about  1 1/2 years.  The grill works great and the ceramic grates are great for cleanup.  I recently got a dishwasher and tried cleaning the grates this way, but when they come out of the dishwasher there is a black sooty film all over the backside of the grates.  I have to wipe with a paper towel and then handwash to get most of the residue off, but it leaves the backside of the grates very discolored - almost as if it has corroded the back of the plates.  Any ideas what is causing this?  The ceramic coated fronts clean up nice, though.