A mounting mat for an exhaust gas treatment device includes first and second major opposite facing surfaces, leading and trailing edges, and opposite lateral edges. At least a portion of one or both of the opposite lateral edges includes an edge protectant that protects the mounting mat against erosion from hot gases encountered during the normal operation of the exhaust gas treatment device. The edge protectant remains flexible after it has been applied to the mounting mat and substantially dried. Also disclosed is an exhaust gas treatment device that includes a housing and a fragile structure mounted within the housing by the mounting mat having the edge protectant deposited thereon. Additionally disclosed are methods of making the mounting mat and an exhaust gas treatment device.