How Much Fumed Silica to Add to Epoxy (Fillets and Fairing)
16 June 2026
Fumed silica (cabosil is the brand; fumed silica is the material) is how you turn mixed epoxy from something that runs off a vertical surface into a paste that stays put. It's the default thickener for fillets, gap filling, and bonding paste. Two questions come up every time: how much do I add, and how do I mix it without a dusty mess or a weak batch.
Shop Aerosil / Cabosil fumed silica
Doing this work for paying customers? Boat Suppliers shop accounts get a flat 20% off catalog. Apply for a Wholesale Pro account.
What fumed silica actually does
It's a thixotropic thickener: a small amount makes resin non-sag without adding meaningful weight, and it keeps almost all of the resin's strength. That's the tradeoff against lightweight fillers, fumed silica makes a strong, hard mix that's a pain to sand; phenolic microballoons and Q-cells make a soft, light mix that sands easily but is weaker. Structural work wants silica. Fairing a big area, you'll usually cut it.
Mix the resin first, then add silica
Mix your resin and hardener to ratio first and get a complete mix, then stir in the fumed silica. Add filler before the hardener is fully blended and you can't tell whether you hit the ratio, and an off-ratio epoxy never fully cures no matter how thick it is. Add the silica in small scoops, stir each in fully before the next; it fluffs up and looks like more than it is, then thickens suddenly.
How much: consistency by use
| Consistency | Behaves like | Use |
|---|---|---|
| Light | Syrup, coats a stick, slow drip | Pre-coat / wet-out before a structural mix |
| Medium | Mayonnaise, holds a soft peak, slowly self-levels | Bonding paste, gap fill on flat or near-flat |
| Heavy | Peanut butter, holds sharp peaks, no sag on vertical | Fillets, vertical gap fill, overhead work |
There's no clean weight ratio, and that's not a dodge: fumed silica is so light it's measured by feel and volume, not a 2% number, and the amount shifts with the resin and the temperature. Expect to add a surprising amount by volume to reach peanut butter, and add it gradually so you don't blow past the consistency you wanted.
Fillets: silica, often cut with microballoons
A structural fillet, tabbing a bulkhead, a stringer-to-hull joint, wants a strong, non-slumping mix that's mostly fumed silica. If the fillet is large and you'll fair over it, thicken first with fumed silica for body, then add some microballoons so the cured fillet sands without a fight. A pure fumed-silica fillet is strong but brutal to fair, and you'll hate it at the sanding stage. For bonding onto cured laminate, make sure your surface prep is right before you apply any thickened mix.
Dust: respirator on before you open the bag
Fumed silica is an ultra-fine powder that goes airborne with the lightest puff. Wear a fitted P100, mix in still air, and don't pour it from height.
The short version
Mix epoxy to ratio, then stir in fumed silica a little at a time to the consistency the job needs: mayonnaise for bonding paste, peanut butter for fillets. Cut it with microballoons whenever you'll have to sand it. Respirator on before the bag comes open. If you run into sticky spots or cure problems, check your ratio and mixing before assuming the filler is the cause.
Doing this work for paying customers? Boat Suppliers shop accounts get a flat 20% off catalog. Apply for a Wholesale Pro account.
Next post
Secondary Bonding: Which Resin Sticks to Cured Laminate (and Which Won't)
Updated on 16 June 2026