Fairing and Filler Calculator
How much fumed silica, microballoons, or other filler to stir into mixed epoxy for a ketchup, mayonnaise, or peanut butter consistency. The answer updates as you type.
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- 5 filler types
- Cups and fl oz
- Matched to jar sizes
Epoxy filler consistency chart
Dry filler to stir into 8 fl oz (1 cup) of mixed epoxy, measured by volume and scooped loose from the jar. Add about two thirds, mix, then add the rest until the mix holds.
| Consistency | Fumed silica | Microballoons | Q-Cells | Glass bubbles | Cotton flock |
|---|---|---|---|---|---|
| Ketchup | 3/4 cup (5.6 fl oz) | 1 1/4 cups (10.0 fl oz) | 1 1/4 cups (10.0 fl oz) | 1 1/4 cups (10.0 fl oz) | 1/4 cup (2.4 fl oz) |
| Mayonnaise | 1 cup (8.0 fl oz) | 1 3/4 cups (14.0 fl oz) | 1 3/4 cups (14.0 fl oz) | 1 3/4 cups (14.0 fl oz) | 1/2 cup (4.0 fl oz) |
| Peanut butter | 1 3/4 cups (14.0 fl oz) | 2 1/4 cups (18.0 fl oz) | 2 1/4 cups (18.0 fl oz) | 2 1/4 cups (18.0 fl oz) | 1 cup (8.0 fl oz) |
| Consistency | Fumed silica | Microballoons | Q-Cells | Glass bubbles | Cotton flock |
|---|---|---|---|---|---|
| Ketchup | 2 3/4 cups | 1.25 quarts | 1.25 quarts | 1.25 quarts | 1 1/4 cups |
| Mayonnaise | 1 quart | 1.75 quarts | 1.75 quarts | 1.75 quarts | 2 cups |
| Peanut butter | 1.75 quarts | 2.25 quarts | 2.25 quarts | 2.25 quarts | 1 quart |
Starting amounts for epoxy at room temperature. Fumed silica weighs only about 50 grams per liter, so the fumed silica column works out to roughly 3, 4.5, and 7.5 percent of the epoxy weight.
How the filler calculator works
Filler is measured by volume because you scoop it from a jar. The calculator multiplies your mixed resin volume by a starting ratio for the filler and consistency you pick, then suggests the jar size that covers the job.
- Mixed resin. Resin and hardener already mixed to the correct ratio, in fl oz, pints, quarts, gallons, or mL. Always mix first, then thicken.
- Filler. Fumed silica or cotton flock for structural bonding and fillets. Microballoons, glass bubbles, or Q-Cells for fairing putty that sands easily.
- Target consistency. Ketchup for coating and sealing, mayonnaise for general bonding, peanut butter for fillets, gap filling, and vertical or overhead work.
The fillers behave very differently. Fumed silica builds a gel, so equal volumes of powder and resin already make mayonnaise. Microspheres thicken only by crowding the resin, so a fairing putty takes about twice the resin volume in dry filler.
Choosing the right filler
- Fumed silica (Cabosil / Aerosil). The strongest thickener and the best sag control. Hard to sand, so save it for fillets and high-strength bonding.
- Phenolic microballoons. Lightweight fairing filler that sands easily. Good for shaping and fairing, not for high-load joints.
- 3M glass bubbles. Light and white, sands easily, and works well for fairing above the waterline.
- Q-Cells. An economical fairing filler that mixes smooth and sands easily.
- Cotton flock. Short cotton fibers for tough structural bonds. Harder to sand than fairing fillers.
Fairing and filler FAQ
How much fumed silica do I add to epoxy?
For a mayonnaise consistency, use about the same volume of fumed silica as mixed epoxy: 1 cup of powder per cup of resin, or a quart per quart. Use about three quarters of the resin volume for a thin ketchup mix and about 1 3/4 times for peanut butter. By weight that is only about 3 to 8 percent, because the powder is so light.
How much microballoons do I mix with epoxy for fairing?
For a spreadable fairing putty, stir in about 1 3/4 to 2 1/4 times the resin volume of microballoons, glass bubbles, or Q-Cells. For 8 fl oz of mixed epoxy that is about 1 3/4 to 2 1/4 cups. These fillers thicken by filling space, so it takes more than most people expect.
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. Mix the resin and hardener to the correct ratio first, then stir in the filler. Once the powder is in, it is hard to blend the hardener evenly, which can leave soft spots that never cure 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. The amounts in this calculator are based on epoxy, so judge polyester mixes by consistency.