what I have done in the past is not go straight up and down on the panel. go at a crosshatch pattern with the durablock, that will take off the high spots and not allow you to make the same groove over and over. then when you think you have it, wipe it down with a wet rag full of wax and grease remover and that will show a shiny surface for a few secs. you will be able to stand back and see any ripples in the finish, just like it was freshly painted.
just an idea that worked for me.