Fillers and bonding

The three fillers

Neat epoxy runs, sands badly and does not fill a gap. Powder fillers are added, each with its own job:

• Fumed silica (Aerosil, Cab-O-Sil): thickens the resin so it no longer runs (thixotropy). The base of glues and pastes.
• Microballoons (hollow glass bubbles): make the mix lighter and easy to sand. The base of fillers and fairing.
• Milled fibre (glass or carbon, optional): reinforces the glue, for joints under load (boxes, inserts).

Basic rules

1. Weigh the resin and hardener first at the maker’s ratio, to the gram. Fillers never count in that ratio.
2. Add the filler little by little while stirring, up to the consistency you need. Makers think in consistency (ketchup, mayonnaise, peanut butter): the percentages below are starting points.
3. Mixing order: West System and TotalBoat add fillers to the mixed resin and hardener. Easy Composites advises adding silica and bubbles to the resin alone, before the hardener (especially with fast epoxies): adding the hardener of a 2:1 system afterwards cuts the silica’s effect by a third. Both work as long as the ratio is weighed without the fillers.
4. Stir for a long time, scraping the bottom and sides of the cup: silica takes several minutes to thicken fully.
5. Make a test batch with your resin: powder grades and densities differ from brand to brand.
6. Small batches, slow hardener: a cup of filled resin heats up fast (exotherm). Spread the mix as soon as it is ready.

Starting doses

As a percentage of the weight of mixed resin (resin + hardener), to adjust to the consistency.

Typical mixes

Bonding the boxes, inserts, handle and plug

Epoxy with slow hardener, 2 to 3% silica so the glue does not run to the bottom of the pocket, and to reinforce the joint, 5 to 10% milled fibre. Butter both the part and its pocket: the glue should squeeze out of every gap. Before that, sand the plastic parts with coarse grit (40 to 80) and degrease them.

Bonding the PVC into the EPS (plate, cross-braces, insert blocks)

For well-fitted parts, the same mix is enough: epoxy and 2 to 3% silica, milled fibre optional. If there is a gap to fill, a microballoon + silica mix fills while staying light, but it is a filler paste: makers keep microballoons for fairing rather than bonding.

Sealing the EPS before laminating

Workshop rule of thumb, by volume: 1 volume of mixed resin, 1 volume of microballoons and ¼ volume of silica. You get a thick mayonnaise, spread with a flexible spreader and scraped off as much as possible. Converted to weight with the powders’ density (about 75 g/l for glass bubbles, 50 g/l for silica), that is about 7% microballoons and 1% silica, within the makers’ limits. A few drops of pigment help to spot missed areas. Count about 100 to 120 g of resin per side for a 6’1 (1.85 m) board.

Common mistakes

• Counting the fillers in the ratio: the resin cures badly.
• Pouring all the filler at once: lumps and air bubbles.
• Mixing a big cup of loaded glue: it heats up and sets before it is in place.
• Bonding to smooth or greasy plastic: sand, then degrease.
• Overloading “to be safe”: a mix that is too dry wets the surfaces poorly and traps air.

Sources

Doses cross-checked between maker data sheets, retailer sheets and a university study, checked in October 2026. Your resin’s and fillers’ data sheets remain the reference.

• Easy Composites — Fumed silica (Aerosil 200)
• Easy Composites — Glass bubbles
• West System — 402 Milled Glass Fibre Blend
• West System 406 Colloidal Silica (CFS retailer sheet)
• West System 410 Microlight (FSL retailer sheet)
• TotalBoat — Milled Glass Fiber
• University of Camerino — Influence of milled glass fiber fillers on the interlaminar fracture toughness of epoxy resin
• Evonik — Aerosil 200 data sheet (tapped density)
• 3M — Glass Bubbles K1 (density)