Fiberglass parts are notorious for poor fitment—and if you’ve ever ordered a set that looked good online but arrived with huge gaps, misaligned edges, and missing flanges, you’re not alone. In this episode of Whatever Wednesday, Sam Kimmel of StealthBuilt tackles the challenge of installing a set of junk fiberglass fenders onto a 1940 Chevy with professional-level precision.
The Problem: Fiberglass Fenders That Don’t Fit
When the project rolled into the shop, it was clear this wasn’t going to be a plug-and-play install. The original steel fenders were damaged and rusty, and the fiberglass replacements came with:
- No inner mounting flange
- Warped shapes and inconsistent curvature
- Severe clearance issues with the car’s ultra-low suspension
Sam’s job? Fix all of that. And do it with smart fabrication techniques that anyone can use in their own garage.
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Step 1: Assessing the Fitment
The first challenge was physical access—the car was so low that getting it off the trailer risked damaging the undercarriage. So Sam worked on the trailer, starting with dry fitting the fenders.
He found major issues:
No structure to actually bolt the fenders to the car
Step 2: Trimming, Modifying, and Shaping
To begin reshaping, Sam:
- Marked and trimmed the outer edge to get closer to flush fitment
- Identified areas where fiberglass would need to be added or reshaped
- Noted the need for a custom flange to bolt the fenders securely
The front and rear fenders both needed creative solutions. Rather than cutting and shortening the fiberglass pieces entirely, Sam made smart compromises that allowed the fender to conform to the custom body lines of the car.
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Step 3: Building a Custom Inner Flange
This is where it gets advanced.
To create a structural flange that bolts the fender to the car, Sam used a clever technique:
- Masking Tape + HVAC Foil Tape
- Prevents resin from bonding to the car
- Creates a clean edge for flange formation
- Fiberglass Laminate Layer
- Applied chopped strand with resin
- Carefully shaped over the taped area to form the base of the flange
- Shaping and Trimming
- Once cured, the flange was trimmed with an air saw
- The fender was test fit and refined for tight tolerances
Step 4: Creating a Fiberglass Paste for Final Bonding
Sam whipped up a custom fiberglass paste by shredding chopped strand mat and mixing it with resin and red hardener (for visibility and mix control).
This paste was used to:
- Fill the edge between the new flange and the fender
- Bond the two parts together with strength and flexibility
- Eliminate the need for thick filler layers later
Once applied, the fender was taped into position, allowed to cure, and then removed to reveal a perfectly fitted fiberglass fender with a strong, reusable flange.
Final Result: OEM-Level Fit on a Custom Build
With the fiberglass modifications complete, the 1940 Chevy now features:
✅ Fenders that match the body lines with tight tolerances
✅ Proper wheel clearance and centered fitment
✅ A flange that allows bolting and unbolting like factory parts
✅ Minimal filler and future-ready for paint and finish
Even better? The fenders are strong, functional, and ready for road use—no more floppy or misaligned panels.
Why Fiberglass Gets a Bad Name—and How to Fix It
This build is a perfect example of why fiberglass fenders have a bad reputation—but also why they don’t have to.
The key takeaways:
- Don’t expect out-of-the-box perfection
- Take your time templating and trimming
- Use quality tools and techniques like flange-building and fiberglass paste
- Avoid relying on filler for large gaps—build the panel to fit
Watch the Full Process on YouTube
Want to see every step in action? Watch Sam’s full walkthrough in this episode of Whatever Wednesday:
And while you’re at it, make sure to like, subscribe, and follow StealthBuilt on social for more wild builds, how-tos, and fabrication madness.
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