A trademark of really nice body work is completely flush body panels. There’s a great way to make sure the panels stay flush as you block sand across them.
It isn’t enough to simply sand across the panels. Panels will move when you sand across them, especially doors. This moving is enough to keep the panels from actually being flush. When a door, for example, pushes in, the quarter panel will be over sanded just a little bit, making it lower than the door.
The solution? Use a small amount of body filler in the panel gaps. It only take about 1” at most, and about 3 of them on each gap. Use this on the front and read edges of doors, the hood gap, deck lid, etc. Leave it in place all the way through the body work process. Put these filler spots in before you start the body work, and try not to take them out until you’re all done with the body work and ready to finish the gaps. And then do the same with blocking the primer.
You can see in the picture below where the arrows are pointed to the body filler. Also, notice they aren’t too close to the corners. You need to have plenty of room to cut them out when you’re done block sanding. 