In this step of the StealthBuilt Audi A6 Widebody Build, Sam Kimmel answers a highly requested question: “What kind of filler are you using—and how?” From mixing techniques to custom consistency adjustments, this episode is all about the art of smoothing, shaping, and prepping custom fiberglass panels for final finishing.

Whether you’re building a widebody kit or fixing a fiberglass panel, this episode is packed with practical bodywork strategies.

What Kind of Filler Does Sam Use?

Sam uses a lightweight polyester body filler, purchased in bulk for cost efficiency. While it comes thick from the factory—ideal for initial build-up—he modifies it depending on use:

• For final coats: He mixes in ~10 oz of liquid polyester resin per gallon to thin it to a “cake batter” consistency.
• For vertical surfaces: He reduces the amount (e.g., 5 oz per gallon) to prevent sagging.

“It’s not officially recommended by manufacturers, but for prototyping work, this works perfectly.”

This technique turns the filler into a self-leveling glaze, reducing sanding and improving surface flatness for final coats.

Taping, Spreading & Sculpting Door Gaps

Sam carefully works around the flare-to-door transition, using:

• Aluminum tape to define panel separation and create clean filler break lines
• A file sander for tight areas
• An air saw was used to surgically split the door panel flare for tight, factory-like gaps

Once cut, the gap is hand-sanded down to a precise ⅛-inch spacing—ideal for future molding and production.

Safety First: Suit Up for the Mess

While Sam is comfortable with filler dust and light fiberglass handling, he always suits up for heavy cutting or grinding. He uses:

• A Shoot Suit (breathable protective gear)
• Respirators and gloves for fiberglass sanding or heavy-duty primer prep

Primer That Lays Like Filler: High-Build Polyester

The primer Sam uses is a high-build polyester primer that:

• Can be brushed or sprayed
• Uses MEKP catalyst, just like fiberglass resin
• Fills pinholes and waves efficiently before blocking

He even experiments with pigment tinting to create a more visible surface. The result? A thick, sandable primer layer that behaves like resin when hardened.

“This stuff gets harder the longer it sits, just like fiberglass. If you want easier sanding, hit it within the first couple of days.”

Tools That Made a Big Difference

Here are some of the tools Sam found most valuable in this stage:

• Cordless file sander – great for tight spaces and hard-to-reach seams
• Air file (longboard sander) – for smoothing long rocker panels
• 90-degree grinder with Roloc disc – for trimming excess fiberglass around flare edges

Sam emphasizes that even budget-friendly tools can have a huge impact on speed and final results in body shaping.

The Final Transformation Before Paint Prep

Once all flares are shaped and primed:

• Tape is removed to reveal final panel contours
• Flare edges are trimmed flush
• Gaps are tested for mold-readiness
• High-build primer is applied in two coats, brushed on for control

Sam plans to flip the car and repeat the process on the opposite side, then wet sand, block, and smooth it all out before tackling the rear bumper.

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Key Bodywork Tips from Part 9

• Thin filler for topcoats with polyester resin for easy spreading and minimal sanding

• Use aluminum tape for clean filler transitions and door gaps

• Brush-on high-build primer saves time and mess vs full masking

• Sand sooner for easier results — these primers get harder with time

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Watch the Full Process

📺 Watch this build stage on YouTube
📸 Get behind-the-scenes tips and tools on Instagram: @stealthbuilt
💬 Drop your bodywork questions in the comments — Sam answers them in future videos!