Deeper surface rust calls for more aggressive treatment. Here, we’re using a 60-grit flap disc on a rotary buffer to dig out a more firmly entrenched section of surface rust. Remember that whenever you’re shooting sparks, you’re removing metal, and that will likely leave low spots in the surface of the panel that you may need to fill later. Also, aggressive grinding drastically increases the likelihood that you will warp the panel you’re working on because of the intense heat generated by the friction between the grinder and the metal. If you are going after deeper rust, at the very least use a flap disc and stop periodically to let the panel cool.