Hollow Fiberglass Wheel Pants From a Reusable Two-Part Mold

Nov 11, 2026 Last reply: 6 hours ago 4 Replies

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Mike_Timber 3 days ago

Most wheel pants on scratch builds start life as a solid balsa block carved to shape, or as store-bought fairings. AviPlanes took a different route here: a handmade balsa master plug turned into a reusable two-part negative mold, producing a hollow fiberglass pant for the 900 mm Pitts S1C. For anyone who has only shaped spats by hand, that workflow looks like a proper step up in finish and repeatability.

The way it is put together is methodical. The balsa plug is shaped, sealed and finished first, then seated into a wooden parting board and sealed with purple modeling clay. The mold itself is built in two stages with white tooling gelcoat and fiberglass, and alignment holes are drilled into the flange before anything is laid up.

The layup is where the two halves earn their keep. Fresh fiberglass cloth soaked in epoxy resin goes into each cavity half, then the mold halves are bolted together and clamped, with spring clamps fusing the wet laminate seam. Popping the mold apart releases a raw, translucent shell, and a Proxxon rotary tool with a cut-off wheel and sanding drum cuts and smooths the lower slot to fit the tire and wheel.

One trade-off worth noting: the part leaves the mold raw and translucent, so the slot work and smoothing all happen after demolding. That said, since the plug-to-mold approach adapts to many small fiberglass and composite parts, the effort put into the mold should pay for itself across future components.

Project by AviPlanes. Watch it on their YouTube channel:

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SawdustBear 3 days ago

The modeling clay seal around the plug in the parting board stands out to me. It reads as a smart call for getting a clean parting line before the gelcoat stages begin, though it clearly adds one more hands-on step to mold prep. Given the flange alignment holes drilled afterwards, the whole setup looks carefully thought through.

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WorksFrost 3 days ago

How do the two mold halves stay lined up when they are bolted together for the layup? With a hollow part, any shift between the halves at the seam would show up in the finished shell, so I am curious what keeps everything registered during the clamping.

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MossPilot_1973 2 days ago

Alignment holes are drilled into the mold flange, and that is what registers the halves. The mold halves are then bolted together and clamped, with the spring clamps fusing the wet laminate seam while everything is held in position. So the registration comes from the flange hardware rather than any guesswork at layup time.

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cweber 2 days ago

Since the plug-to-mold technique adapts to many small fiberglass and composite parts, it would make sense to shape a few extra plugs while the setup is still on the bench. Other fairings on the same Pitts could come out of matching molds, and one reusable mold set would then serve several components instead of just the wheel pants.

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