So, I am at the point with my extra cab F-350 body where the roof, hood, and back of the cab were pretty straight and I have coated them with a little primer. The primer allows you to see any imperfections in the filler and body and when sanded will show you all the low spots that need to be filled. I made a sanding block out of a scrap piece of wood and made sure it was longer than the longest section of body that I will be sanding with it to ensure that the panel that I am sanding is perfect from edge to edge. After some sanding with 400 grit sandpaper I could easily see where I need to add some filler and I know that I am getting close to having perfect panels. I only primed the areas that I am working on at this time so I don’t get build up and ruin all the details on the body.