In this part of the StealthBuilt Audi A6 Widebody Build, Sam Kimmel dives into the final form-shaping of the rear bumper, experimenting with diffuser designs, center brake lights, and even prepping the panel for mold-making. This chapter blends high-speed creativity with deep craftsmanship—and a whole lot of cardboard.
Mocking Up Rear Diffuser Designs with Cardboard
Sam kicks things off with a classic StealthBuilt move: cardboard. Lots of it.
- Multiple diffuser blade variations are mocked up in cardboard
- A race-inspired center brake light is temporarily fitted and repositioned
- All mockups are posted to Instagram and TikTok for real-time community voting
“I make everything bigger than I need, then trim it back. That way you’re never stuck with something too small.”
This low-cost prototyping style lets Sam test ideas quickly and refine based on gut feel—and audience input.
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Strengthening the Bumper with Extra Fiberglass Layers
Originally fiberglassed with only 2 layers, the bumper lacked the rigidity needed for serious bodywork. Sam peels off the mockups and re-glasses the entire lower section with multiple new layers of mat:
- Chopped strand mat is used for its flexibility
- Wet-out is done flat on cardboard, then transferred to the bumper
- Sam explains: working against gravity wastes time and resin, especially for vertical or overhead work
“Trying to fiberglass upside down is just asking for a mess. Wet it out flat—then apply.”
When Big Pieces Fail: Cut It Up and Keep Going
Sam demonstrates what happens when he tries to slap a large fiberglass mat onto the vertical diffuser: gravity wins. It fails, starts peeling off—and he slices it up into strips and reapplies it.
Lessons like this are gold for other builders:
- Big pieces sag under their own weight
- Smaller strips are easier to contour
- Failures are part of the process—just adapt and keep going
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Laying the Foundation for Molding
While this bumper won’t stay on the car, it will serve as the buck to create molds. That means:
- Strength matters more than finish
- Filler usage is heavy—over a gallon, just to get the perfect shape
- Door gaps, exhaust clearances, and creases all get tuned to perfection
“I would never use this much filler on a final car—but for a mold buck? Doesn’t matter. It just needs to be perfect in shape.”
MDF Diffuser Fins for the Finishing Touch
With time ticking toward their first car show appearance, Sam hand-cuts MDF diffuser blades, router-rounds the edges, and shapes them up fast. The goal? Show up to the event with something that looks near-final, even if it’s still in-progress underneath.
These kinds of DIY hacks are essential when pushing timelines and dialing in looks under pressure.
Reflecting on the Exhaust Tip Design
Originally angled, the exhaust tip orientation was a hot debate on social media. Even after shaping the bumper around them and doing bodywork, Sam admits:
“They looked okay. But I’m not after okay. I’m after something cool.”
He reveals he’ll be recutting the rear bumper again to redesign the exhaust exit.
That’s real craftsmanship: not settling, even after hours of labor.
The Hardest Part of the Build? The Back Bumper.
Sam doesn’t sugarcoat it: the lower rear bumper is the most difficult section:
- Awkward angles
- Vertical fiberglass work
- Tight spacing around the exhaust
- Time-consuming shaping and sanding
But it’s also the most visible, and the effort pays off in visual impact.
Key Tips From Part 10
- Use cardboard to prototype everything—fast, cheap, and flexible
- Reinforce fiberglass mockups before bodywork
- Wet fiberglass on the floor, then apply it to vertical surfaces
- Don’t be afraid to revise your design mid-process
- Focus on shape now—surface perfection comes later with molds
Want to See the Build in Action?
📺 Watch the full rear bumper episode on YouTube
📸 Vote on the next part of the build at @stealthbuilt on Instagram
🛠️ New merch dropping soon on Etsy — hats, shirts, and more for DIY car builders
