Precise gaps are a crucial part of doing great body work. The way panels fit and flow together can make or break the look of a car, and at best can take the project to a whole new level.
To help simplify the process, this is an overview of each step. The brief description of each step is under each picture.
Step 1 is assuming all body panels are already in place and adjusted. The point of this step is to make sure all the panels are flush and flow together. Use a long piece of aluminum flat stock or c-channel, this will show you high areas to be shrunk, and will show you where to move panel edges out to. All the panel edges must touch the aluminum. View the flat stock as a preview to where the finished body filler will be.
Step 2 can somewhat go back and forth with step 1. Typically, when you move a panel edge out, the gap will open up. So Step 2 is to weld and grind the gaps to the width you want and make sure the metal edge is completely straight. Using a large hand file works great. Always continuously check the edges with the flat stock. Use the flat stock as a guide, a lot.
Step 3 is body work. While you’re doing body work, you can completely ignore the gaps. First establish the large main areas, and then finalize the body lines. But, don’t even think about the gaps. Just try to keep the filler out of them. It’s okay if filler bridges over the gap though. Use the flat stock as a guide here too, a lot.
Step 4 is to carefully cut the gap back open with a cut off wheel. Run it slow, take your time. If you bump into metal, stop and adjust direction. Most importantly, run it very slow. This step should absolutely not cause any damage.
Step 5 is to clean up the gap and prep it for filler. Grind back any loose filler, and make sure there’s no air pockets in the body filler against the edges of the panels. Taper the filler back, and you can sand the edge of filler by hand with 80 grit so it’s not a sharp edge. Blow it off good, the gaps should be well prepped ready for body filler after this step.
Step 6 is to use the appropriate thickness of polypropylene plastic to put in the gap opening, and pull it tight against the door. There should be a void between the plastic and the quarter panel. You will squeeze body filler in here, and let it dry. When you set the gap thickness in metal, be sure this size plastic will fit square in the gap, all the way in.
Step 7 is to pull the plastic out after the filler is dry, and block sand the little bit of filler flush. This will leave you with a perfectly consistent and square gap.