Tips for 3D printing big car body panels that nobody warned me about

My expirience with the assembly phase was pretty rough until I figured out how to manage the stress points where the sections meet. Even with the flat tabs, the material around the joint often flexes a bit when the glue dries, which opens up little gaps that are impossible to touch with a standard filler. I ended up using a two part epoxy mixed with some fine grit filler to build up the seam, rather than just relying on the plastic tabs themselves. Getting the surface smooth enugh for priming was the real job. I found that if you are using a TPU mix, you need to run the final sanding thru a series of grits, going from a 120 down to 400, and then a final pass with something very fine, like 800 grit. If you skip that fine grit step, the clear coat just won't flow right over the texture. I spent three weeks just on the finishing of a quarter panel before it looked remotely like it would survive outside.

I also had issues with the way the seams interacted with the underlying body structure. It required me to sand a very shallow channel into the original metal just to ensure the printed section sat flush and didn't introduce any stress concentration at the mounting point. It's far more work than you think once you get past the printing stage.

Gotta push back on cutting a channel into the original metal to seat the panels. That sounds like a good idea until you're thinning out a sill or a quarter panel edge that's allready seen some rust, and now you've got a weak spot hiding under all that filler. I just shim the printed section out with plastic spacers and pull the seam over with filler instead. Takes longer with the sanding but the body stays untouched. Am I missing something, or did your printed parts actually sit that far proud of the surface?

I found the printed panels often lack rigidity, expecially the larger flatter pieces. Before I glued everything togehter, I filled the main internal cavities with a stiff foam core. It adds weight but it completly stops the panel from flexing under its own weight and prevents stress from building up near the mounting points. I also learned that you have to let the adhesive cure way longer than the instruction book suggests, especially when working on the deep seams.

The foam core trick is legit, I did the same on a door skin and it went from floppy to solid almost overnight. One thing I learned though, use the non-expanding stuff. I used regular expanding foam on my first panel and it swelled enough to bow the whole thing outward while it cured. Had to cut it all back out and redo it. Since then I pack it in layers and it behaves.

Update: tried the foam core idea on the roof section that was still flexing and it made a huge difference. Used the non-expanding type after reading what happened with the swelling stuff, poured it in thin layers so it wouldn't push anything out of shape. Panel is solid now and the joint gaps stayed closed. Glued the whole roof together last night.