Creating fillets is somewhat of an art. Mix the filler too loose, and the fillet sags; mix it too thick, and it resembles spreading dry bread dough. Fillets are used to transition fiberglass from one part to another, and that transition is important because fiberglass loads do not like to follow sharp 90° paths. A good consistency is important as it maintains a dimensional radius, allowing the glass to contour through when applied to a non-cured, freshly applied fillet.
There are two basic filler categories: low density and high density. Glass bubbles, microballoons, and Q-Cel fall into the low-density category. Cabosil and Aerosil are brand names for fumed silica and fall into the high-density category. Milled fiber and cotton flox also fall into the same high-density category; they are used in structural applications, though not necessarily for their thixotropic qualities. There are applications that require a solid flox fillet, a solid Cabosil fillet, or—if looking for maximum weight savings—a solid low-density filler fillet.
Cabosil is a thixotropic filler, meaning it increases the thickness of the resin. This prevents a fillet from sagging and adds a structural component, but because it is a high-density filler, it also adds weight. When Cabosil is combined with a low-density filler, the result is a more workable material. Straight Cabosil will make a stiff, dense, heavy fillet that may be required in structural situations. Milled fiber can also be added for structural integrity but is not typically needed in many fillets.
A high-density fillet mixed thick will lay down and form a shape that the glass conforms to without deforming the fillet. A low-density filler can do the same, but the window of opportunity for the proper amount of filler is limited. If you use too much low-density filler by itself, it will not be spreadable; too little, and it will sag. Furthermore, too much of any filler will compromise the bonding of the fillet to the substrate. It robs the fillet of resin, resulting in a dry fillet with poor bonding ability.
Adding a small amount of high-density filler, like Cabosil, to a low-density fillet application increases the mixing latitude. This allows you to vary the low-density filler amount and still produce a usable fillet.
Since many fillets are basic transition fillets, we are skewing this toward the “common” fillet—one used to fill a junction and allow the glass to make a transition. A simple recipe for this non-sagging, spreadable fillet involves adding a high-density filler (Cabosil) until the viscosity is somewhat higher than the base resin viscosity but less than a mayonnaise consistency. At this point, add a low-density filler until the mix reaches a peanut butter consistency or thicker. This creates a very usable mix.
Adjusting the amount of Cabosil at the beginning to be thicker than mayonnaise will make a firmer, less dry fillet. This approach works for any quantity of resin. If you mix batches frequently, you may wish to develop a known quantity recipe—rather than relying on arbitrary “mayonnaise” consistencies—for repeatable mixes.
A good learning tool is a waxed, engineering-grade aluminum angle (the hardware store variety). Mix small batches with different ratios and have a “fillet pulling party” to see how they act in application and vertical hang. If it does not want to stick and stay put when applied, it is most likely too dry … add less low-density filler and more Cabosil.
