A free radical curable automotive body primer is provided that has a color change corresponding to the progress of the cure process until a dry to sand condition has been attained and an internal colored guide coat that does not change color given its exposure to ambient air and as a result, has a different color on the surface of the primer than when in bulk, allowing for the detection and removal of low spots or other imperfections from a vehicle body by additional sanding of the inventive primer to remove oxidized guide coat colorant surface regions. The cure color change is achieved through resort to at least one color changing dye that reacts with a free radical cure initiator during the vehicle body priming process and changes color to indicate when the primer composition has achieved a level of cure so as to be dry enough to sand.