I just completed my air tests with the "TimeRanger",with the 8" Coil.I found that the spray-on Truck bed coating gave these results:At about 6 to 7 inches with the machine on the all metal mode,and holding any coin...ie: nickle,,dime..quarter. on edge to the coil,which gives the least surface area to pick up,I recieved an audible tone.I also held these same coins flat,giving the most surface area to the Coil and recieved readings at approximately 10 inches.I do however think that on my 4 and 10 inch Coils I will put on two even light coats and call it good as this first Coil was coated a bit heavy.My recomendation,try one coil fixed this way,,in the field, before you do all your coils,to see how it works in the real world so to speak.I was pleased with the air tests and hope the weather gets better soon so I can try my new detector with my newly preparred Coil.,,,,HappyHunting!!!!!!!!!!!!!!!!!!!!!!!!!1