Fairing & Filler Calculator
Work out how much filler to stir into your resin to hit the right consistency, whether you are making a structural fillet with fumed silica or a sandable fairing compound with microballoons. Pick your filler, enter how much resin you are mixing, choose a target consistency, and get a starting amount plus the right jar to add to your cart.
How the filler calculator works
Thickening resin is about consistency, not a fixed recipe, because fillers differ in how much they bulk up a given volume of resin. The calculator starts you at a sensible amount for your chosen filler and target consistency, then you fine-tune by eye. Here is what each input does:
- Resin you are mixing. The volume of already-mixed resin and hardener. Always mix to the correct ratio first, then thicken.
- Filler. Fumed silica (Cabosil) for strong structural work, or a lightweight fairing filler (microballoons, glass bubbles, Q-Cells) when you want it to sand easily. Cotton flock is a tough structural option.
- Target consistency. Ketchup for coating and sealing, mayonnaise for general bonding, peanut butter for fillets and gap filling that will not sag.
Choosing the right filler
- Fumed silica (Cabosil / Aerosil). The strongest, most structural thickener. Thixotropic and stops sagging fast, but it is hard to sand, so save it for fillets and high-strength bonding.
- Phenolic microballoons. A lightweight fairing filler that sands beautifully. Great for shaping and fairing, not for high-load joints.
- 3M glass bubbles. Light and white, sands easily, and works well above the waterline for fairing.
- Q-Cells. An economical fairing filler with easy sanding.
- Cotton flock. A tough structural bonding filler when you need strength but a little more body than fumed silica alone.
Fairing and filler FAQ
How much fumed silica do I add to epoxy?
For a mayonnaise consistency, start with roughly one fifth the volume of your mixed resin in fumed silica, then add more a little at a time until it holds. For a thick, no-sag peanut butter you will use a bit more. Enter your resin volume above for a starting amount.
What is the difference between fumed silica and microballoons?
Fumed silica (Cabosil) is a strong, structural thickener that is hard to sand, so it is used for fillets and high-strength bonding. Microballoons and glass bubbles are lightweight fairing fillers that sand easily, so they are used for shaping and smoothing, not for load-bearing joints.
What consistency should my filler be: ketchup, mayonnaise, or peanut butter?
Ketchup (a thin syrup that pours) is for pre-coating and sealing. Mayonnaise (holds on a horizontal surface, sags a little) is for general bonding and laminating. Peanut butter (thick and will not sag) is for fillets, gap filling, and vertical or overhead work.
Do I add filler before or after the hardener?
After. Always mix the resin and hardener to the correct ratio first, then stir in the filler. Adding filler before the resin is fully catalyzed can leave you with putty that never cures properly.
Which filler is best for fillets versus fairing?
Use fumed silica or cotton flock for structural fillets where strength matters. Use microballoons, glass bubbles, or Q-Cells for fairing, where you want a light, easy-to-sand surface. Many builders use a structural filler for the fillet and a fairing filler for the smoothing coat on top.
Can I use these fillers with polyester resin and gelcoat?
Yes. Fumed silica, microballoons, glass bubbles, and Q-Cells all work with epoxy, polyester, and vinyl ester resins. Fumed silica is also commonly added to gelcoat and resin to stop it sagging on vertical surfaces.