Generally, even without sanding down, I've almost always had a solid plate connection using locking washers as they dig through the finish to some degree (testing for resistance between the screw and ground), however to be extra sure there's a good contact, I like to use a dremel tool with a metal bit to sand away the area.
Another note, powdercoaters often handle electronic equipment and have little heat resistant stickers they can put wherever you have a ground connection, like this: http://www.diyaudioblog.com/2013/04/audio-note-dac-21-level-b.html