Instead of painting gray paint in grooves, I tried this trick. Take sandpaper fold it a few time to create roughly a 1/16 to a 1/18 inch edge. Use sandpaper to rough up surface inside grooves, follow contour of groove. The roughed up glossy Black ABS turns to a flat gray. You can control thickness of line, fade line to look like weathering. See sample picture of my armor. Let me know what you think.