My father is a do it quick DIY'er and usually ends up making more of a mess than when he began. He patched some drywall using regular compound. He then sanded each spot by hand and left awful markings and grooves. Not only did he do a bad job he then painted the entire room including the newly patched areas. How can I repair these spots? Would an electric sander and course sandpaper work, or do I need to cut out the portion of wall and start from new?