Filleting

A fillet is a rounded, structural compound applied to an internal corner or joint created when substrate materials are joined together. A fillet serves two primary purposes: increasing the surface area for bonding while also creating a smooth radius that allows fiberglass cloth or fabric to transition around sharp angles without lifting or creating air pockets.

When filleting as a part of an initial build process, the best practice is to utilize the same resin type with which the component is being constructed. For retrofit and repair work on post-cure resin systems, epoxy will achieve the best bond (even if the part was originally constructed utilizing a polyester or vinyl ester resin system).

Composition and Chemistry

Filleting compounds are created by mixing a base resin (epoxy, polyester, or vinyl ester) with appropriate fillers to achieve a desired consistency and strength.

These fillers commonly include:

• Milled Glass Fibers: significantly increases the structural integrity and tensile strength of joints
• Wood Flour: adds density, bulk, and a natural color match for wood projects
• Phenolic Microballoons: lightweight spheres often used in wood construction for its distinctive reddish-brown coloration; added in small amounts, as it is mainly utilized in this application for color and texture
• Cab o Sil: a thixotropic agent to thicken the resin and prevent sagging on vertical surfaces; added to the mixture last

Application and Finish

Unlike fairing compounds, which are designed for aesthetic smoothing and easy sanding, fillets are primarily structural. As they are often reinforced with high-strength fibers, they can be difficult to sand once cured. Consequently, filleting compounds are applied as a “bead line” and smoothed out with specifically-shaped applicators (round-edged) while still “wet” and overlaid with resin-saturated fiberglass cloth or fabric tape. When completed properly, this process minimizes or eliminates the need for post-cure refinement.