Yes, you can use a grinder to grind down a damaged section of a steel coil. However, it's crucial to use the appropriate type of grinder designed for metalworking and to ensure safety precautions are taken, such as wearing protective gear. The grinder should be capable of handling the hardness of the steel and should be operated by someone experienced with such equipment. Always follow the manufacturer's guidelines and safety standards to prevent accidents and ensure the effectiveness of the grinding process.